Support stripping protobuf-java jars in java_export when using protoc via toolchain

[thirtyseven]

Fixes #1480.

This PR should have tests, but I'm not sure of the best way to add them since ideally we want to run tests/integration/java_export in both toolchain and non-toolchain mode (with and without --incompatible_enable_proto_toolchain_resolution) and this doesn't seem easy with the current test suite design. Would like validation about this approach before committing to adding support for this to the test suite.

[shs96c]

This is what the excluded_workspaces attribute of java_export is for. Can't we just add the workspace to the default list?

[thirtyseven]

This is what the excluded_workspaces attribute of java_export is for. Can't we just add the workspace to the default list?

When using the Java proto lang toolchain, the protobuf runtime jar can come from any workspace. For example, in our repo we have it set to be @maven//:com_google_protobuf_protobuf_java and in the toolchains_protoc example it's @protobuf-java//jar.

I updated the linked ticket with a bit more detail.

The alternative would be to tell users to manually specify excluded_workspaces for every java_export when using a Java proto_lang_toolchain, but that seems not very user-friendly and not in the spirit of toolchains.

[shs96c]

LGTM. Thank you!