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

Summary:
Introduced a `version` config field that must be specified and match
`config.CurrentVersion`.

Changed `config.ReadConfig` to use `yaml.UnmarshalStrict` to ensure that
errors are surfaced when unknown/bad fields are present in the given
YAML config. A smaller `config.VersionConfig` is now unmarshaled first
to prevalidate the new `version` field before the entire config is
parsed.

Fixes T191460

Test Plan:
Run `go test ./...`. Run `blubber` against some configuration containing
invalid fields and ensure that it surfaces a YAML error.

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

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Maniphest Tasks: T191460

Differential Revision: https://phabricator.wikimedia.org/D1021

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:
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

Establish phases within Docker compiler to allow providers (apt, npm,
etc.) to inject their own run/copy instructions into to the Dockerfile
compilation process while leaving the compiler agnostic to the providers
themselves.

The instructions and phases are also generalized to leave room for
alternative compilers should they be needed in the future (e.g.  aci
support via acbuild) but also as a general design constraint to leave
compiler implementation concerns out of providers.

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.