We used to pass only the direct classpath to javac when compiling. Now that we use errorprone, we've switched to passing the full classpath (required by errorprone - bug 804331).

Besides having targets no longer always specifying correct deps, this actually leads to confusing errors, such as desugar failing (it still uses direct classpath):
 * E.g.: https://chromium-review.googlesource.com/c/chromium/src/+/1230059/1)

There is also nothing in our build that validates the deps of prebuilt jars, which can lead to proguard / r8 failing (or more often, working by accident due to an unrelated dependency adding the missing one). 
 * E.g. https://bugs.chromium.org/p/chromium/issues/detail?id=872904#c17

We should add to our build system some validation that direct deps are correct. Ideally, it should work for both java_library and java_prebuilt.

One idea is to put it in bytecode_processor.py, except that it uses the full classpath... Perhaps we could pass it both the direct and full classpath so that it can ensure all directly references are in the direct classpath:
https://stackoverflow.com/questions/1983839/determine-which-jar-file-a-class-is-from