Make Java source root configurable

[cushon]

Bazel hard-codes some conventions about Java source roots:

bazel/src/main/java/com/google/devtools/build/lib/rules/java/JavaUtil.java

Line 79 in 3aedb2f

int rootIndex = path.getFirstSegment(ImmutableSet.of("java", "javatests", "src"));

This is used e.g. to infer java_test.test_class and to relativize java_library.resources.

We should make the source root configurable to support other repository layouts.

[cushon]

@lberki pointed out offline that the configuration for this would ideally be per-WORKSPACE, rather than a top-level flag or part of the toolchain.

[petroseskinder]

Thank you for filing this @cushon. Also discussed with @katre, regarding the matter. Internally at Two Sigma, the change that we have made is simply adding "testsrc" to the list, and then adding a set of tests in the JavaUtilTest.java class that verified that testsrc worked.

[cushon]

I'm not sure exactly what the per-WORKSPACE config mechanism will look, and supporting testsrc seems like a good idea in the interim.

Do you want to open a PR for 2a8ff16 and db19ea1?

[lberki]

Yeah... I'm not sure we should have a knob for this in the first place, and I'd rather have a simple and almost correct fix (i.e. the above two commits) than a complex and almost correct one.

[petroseskinder]

This scratches my itch, so it is fine with me if this is not tunable.

[lberki]

Not in the short term -- I'd love to, but there probably won't be enough time before next ~June. It's not a trivial change...

[BlackIsTheNewBlack]

@lberki pointed out offline that the configuration for this would ideally be per-WORKSPACE, rather than a top-level flag or part of the toolchain.

It is fine to have a default per-WORKSPACE, but please consider that many projects are put together under the same workspace, some even from different companies, so it should at least be configurable per-BUILD file. It's fine to have standards, but the configuration should fit the status-quo, not viceversa.

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