Polish Scala `repl`

[stuhood]

Currently the scala repl:

1. defaults to using python unless repl --shell=scala is specified (even if the python backend isn't enabled)
2. fails if a target has resources dependencies (likely due to launching for the transitive dependencies, rather than using the Classpath helper for only the roots).

[tdyas]

And we may want to consider how to default to ammonite as well: https://github.com/pantsbuild/pants/pull/13838

[tgolsson]

Just hit 1) for my OCI backend; and found this in the Pants sources: https://github.com/pantsbuild/pants/blob/ae520ccfc86378dac3bbc651e322c41e8cdfaa05/src/python/pants/core/goals/repl.py#L109-L111 Did you think anything about how to solve it in a generic way?

[tdyas]

The ReplImplementation union could be modified to be more generic in a way similar to how other goal-related unions were: Have ReplImplementation subclasses declare a specific list of FieldSet types which apply for that ReplImplementation. This would allow the repl goal core rules to determine whether targets are compatible with a specific repl implementation.

[tdyas]

For example, TestRequest defines field_set_type for subclasses to specify which FieldSet class applies to the test implementation. https://github.com/pantsbuild/pants/blob/9e95a8e154b85e31fa8907117b36e717c99bf206/src/python/pants/core/goals/test.py#L312

[benjyw]

Fixing 1) (defaulting to the Python REPL even when your current target universe is all JVM) may relate to https://github.com/pantsbuild/pants/issues/16445 - if Pants gains some concept of "realm of interest" that allows it to know which languages/runtimes/toolchains are relevant to the current target set.