Java: use java_source vs java_sources

[Eric-Arellano]

Because Java is experimental, we can fully implement https://docs.google.com/document/d/1HpJn2jTWf5sKob6zhe4SqHZ7KWBlX4a6OJOwnuw-IHo/edit#heading=h.7dq13i2i8pzd.

Already, Java is using target generation:

pants/src/python/pants/backend/java/target_types.py

Lines 48 to 61 in 03c2668

	@rule
	async def generate_targets_from_junit_tests(
	request: GenerateTargetsFromJunitTests, union_membership: UnionMembership
	) -> GeneratedTargets:
	paths = await Get(SourcesPaths, SourcesPathsRequest(request.generator[JavaTestsSources]))
	return generate_file_level_targets(
	JunitTests,
	request.generator,
	paths.files,
	union_membership,
	# TODO(#12790): set to false when dependency inference is disabled.
	add_dependencies_on_all_siblings=True,
	)

Right now, java_tests is generating java_tests. What we really want is to generate java_test (singular). The fix is going to look almost exactly like #12957, only that you can rename the alias for the targets to what we want, whereas in that PR we have to use some hackery to keep the old target names. tl;dr:

• define new targets for java_source and java_test
• rename java_library to java_sources & update help
• update production call sites to always operate on java_source and java_test. Nothing in prod should be using java_sources and java_tests except for the target generation rules and tailor.py. It's totally fine for tests to keep using those target generators for less boilerplate.

No need to implement the overrides mechanism yet.