Support scala_artifact

[alonsodomin]

This is an initial stab at fixing #12969 using target generation to determine the actual artifact name.

It also introduces a breaking change in the excludes field of jvm_artifact to be able to distinguish between which of the mentioned exclusions are Scala artifacts (and hence require name mangling based on Scala version) are which ones aren't.

For example, the following BUILD:

scala_artifact(
  group="com.example",
  artifact="test",
  version="1.0",
  excludes=[
    scala_exclude(group="com.example", artifact="test_scala_exclude"),
    jvm_exclude(group="com.example", artifact="test_jvm_exclude")
  ]
)

Would generate the equivalent to:

jvm_artifact(
  group="com.example",
  artifact="test_2.13",
  version="1.0",
  excludes=[
    jvm_exclude(group="com.example", artifact="test_scala_exclude_2.13"),
    jvm_exclude(group="com.example", artifact="test_jvm_exclude")
  ]
)

For a Scala version of 2.13.x.

This approach could be extended in the future to support cross platform builds to ScalaJS and/or Scala Native.

[stuhood]

Thanks a lot! This looks great.

[stuhood]

Regarding the breaking change: would it be possible to support both exclusion field values for a deprecation period, with the string version always assumed to be a jvm exclusion?

[alonsodomin]

yeah, it makes sense to go through a deprecation phase.

I thought I could have a BUILD file fix rule that could replace the old one with the new one. But it seems I need to invest more time in understanding the current API for that.