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