Refactored `copies` configuration to allow for greater control over when
and which both local build context files and variant artifacts are
copied into the target image.

The new configuration introduces a "local" keyword to signify when a
`copies` entry should apply to the files from the local build context as
opposed to files from another variant's image during a multi-stage
build.

  variants:
    build:
      copies:
       - from: local
         source: ./src
         destination: .

Note that with this change, the user must now explicitly define whether
_any_ files should be copied in from the local build context. None will
be copied in by default.

To help keep configurations succinct, especially considering this new
requirement, and to approximate the old `copies: variant`, a shorthand
format and sane defaults for `source` and `destination` (depending whether
`from` is "local" or a variant name) were implemented.

  variants:
    build:
      copies: [local]
    development:
      copies:
       - from: build
       - from: local
         source: ./config.dev.yaml
         destination: ./config.yaml

The shorthand:

  copies: [ref, ...]
  # is equivalent to
  copies: [{ from: ref }, ...]

And the following defaults are used when no `source` and `destination`
are specified.

  copies:
   - from: local
   # defaults to
   - from: local
     source: .
     destination: .

  copies:
   - from: variant
   # defaults to two entries
   - from: variant
     source: /srv/app # the lives.in dir
     destination: /srv/app
   - from: variant
     source: /opt/local # the shared lib dir
     destination: /opt/local

Bug: T211625
Change-Id: I4c4217905afc0762b6bd66ed594d43cc0486e3e2

Uses the github.com/ghodss/yaml library to convert YAML to JSON before
unmarshaling for the purposes of supporting YAML and JSON input while
converting to only support JSON internally.

Bug: T207694
Change-Id: I00668014907e9ea54917f5d5067cac08d0668053

All import paths and other references to the previously Blubber repo in
Phabricator have been changed to point to the new Gerrit project URL.

Note that building or testing against this change will require you to
move your working directory to `$GOPATH/src/gerrit.wikimedia.org/r/blubber`.

Contribution documentation was updated to reflect the changes, and the
`.arcvendor` submodule has been removed.

Bug: T200452
Change-Id: I3ab23e420b2825e86e2bd7c9b3da9d4de23edaa1

Summary:
The implementation of D984 did not include enforcing ownership for
`build.CopyFrom` instruction and so artifacts copied from one image to
another via `copies:` were problematically owned as root.

In order to fix this behavior:
 1. `config.ArtifactConfig` `build.CopyFrom` instructions are now
    injected duration `build.PhaseInstall`
 2. `config.VariantConfig` calls `build.ApplyUser` for these artifact
    instructions as well using the `runs.as` user
 3. `build.CopyAs` was refactored to wrap any `build.Instruction` which
    should only really be used with `build.Copy` or `build.CopyFrom`.

Test Plan:
Run `go test ./...`. Run `blubber` against configuration with a variant that
uses `copies` and verify that the `COPY --from` instructions also include a
`--chown` flag.

Reviewers: thcipriani, mmodell, hashar, #release-engineering-team, demon

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D1002

Summary:
Implemented a validation system using the
`github.com/go-playground/validator` package, extending it with custom
validation tags, and implemented translation of validation errors into
somewhat human-friendly messages.

Fixes T175186
Depends on D845

Test Plan: Run the unit tests and try running blubber against some bad config.

Reviewers: thcipriani, hashar, Jrbranaa, Joe, #release-engineering-team, mobrovac

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Maniphest Tasks: T175186

Differential Revision: https://phabricator.wikimedia.org/D868

Summary:
Wrote inline documentation for all the things.

Fixes T168000

Test Plan: Execute `godoc -http=:6060` and proofread all the things.

Reviewers: thcipriani, hashar, #release-engineering-team, demon

Reviewed By: thcipriani, #release-engineering-team, demon

Tags: #release-engineering-team

Maniphest Tasks: T168000

Differential Revision: https://phabricator.wikimedia.org/D841

Summary:
Now that our Phabricator instance handles `go get` requests, we can use
a path without the `.git` suffix as the canonical import path.

Test Plan: Run `go test ./...` and `go build`.

Reviewers: thcipriani, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D780

Summary:
Support a `copies` variant config entry that will result in a
multi-stage build, copying both shared library files and application
directory from a previously defined variant. This is essentially a
shorthand for two `artifacts` entries that are likely to be idiomatic to
multi-stage build/prod configurations.

Defined a new abstract `build.CopyFrom` instruction and corresponding
`docker.DockerCopyFrom` instruction and refactored the writing of these
Dockerfile lines to be accomplished using an `InstructionsForPhase`
method on `config.ArtifactsConfig`.

Implemented new support for `copies` configuration in
`config.VariantConfig` and an `InstructionsForPhase` method that returns
`build.CopyFrom` instructions for both the shared library and
application directories.

Fixes T174622
Depends on D759

Test Plan:
Run `go test ./...`. Run `blubber blubber.example.yaml production` and ensure
the right `COPY --from` lines are included for the final stage.

Reviewers: thcipriani, mobrovac, hashar, mmodell, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Maniphest Tasks: T174622

Differential Revision: https://phabricator.wikimedia.org/D768

Summary:
One of Golang's "advantages" is not quibbling over style. To this end
the `go fmt` command exists. This is the result of me running:

    find . -name '*.go' -exec go fmt {} \;

Test Plan: built package, ran tests

Reviewers: dduvall, mmodell, #release-engineering-team

Reviewed By: dduvall, mmodell, #release-engineering-team

Subscribers: hashar

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D694

Reorganized config type declarations.

Added `RunConfig` for declaring working directory and unprivileged
runtime user. A system for enforcing restrictions should probably be
implemented for this.

Implemented Dockerfile compiler, supporting multi-stage output in cases
whether artifacts from other variants are configured.