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