Bazel: add support for DirectoryFileObject

[olafurpg]

Previously, the scip-java Bazel support only supported SimpleFileObject in the Java compiler API and it crashed when encountering other types of file objects like DirectoryFileObject, which are used in some Bazel builds in the wild. This commit adds support for DirectoryFileObject.

Additionally, this commit adds more detailed documentation on how to use scip-java with Bazel.

Test plan

Green CI.

Manually tested there was no regression in the steps below

❯ bazel build //... --@scip_java//semanticdb-javac:enabled=true
INFO: Build option --@scip_java//semanticdb-javac:enabled has changed, discarding analysis cache.
INFO: Analyzed target //src/main/java/example:example (0 packages loaded, 843 targets configured).
INFO: Found 1 target...
Target //src/main/java/example:example up-to-date:
  bazel-bin/src/main/java/example/libexample.jar
INFO: Elapsed time: 0.152s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action

❯ bazel run @scip_java//scip-semanticdb:bazel -- --sourceroot $PWD
INFO: Build option --@scip_java//semanticdb-javac:enabled has changed, discarding analysis cache.
INFO: Analyzed target @scip_java//scip-semanticdb:bazel (0 packages loaded, 863 targets configured).
INFO: Found 1 target...
Target @scip_java//scip-semanticdb:bazel up-to-date:
  bazel-bin/external/scip_java/scip-semanticdb/bazel.jar
  bazel-bin/external/scip_java/scip-semanticdb/bazel
INFO: Elapsed time: 0.184s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/external/scip_java/scip-semanticdb/bazel --sourceroot /Users/olafurpg/dev/sourcegraph/scip-java/examples
INFO: Build completed successfully, 1 total action
running: bazel query kind('.*_import', @maven//...) --output=proto
done: /Users/olafurpg/dev/sourcegraph/scip-java/examples/bazel-example/index.scip

[Strum355]

We're constrained by having to use only public APIs of the Java compiler

Is this a bazel restriction? I dont think this is true for scip-java, we almost definitely use non public parts

[olafurpg]

Not a bazel restriction. We try to limit non-public API usage as much as possible.