…ort of #5901)

Maven does not include transitive runtime dependencies in the
compile-time classpath (testing shows Gradle 4 does; docs say
Gradle 5 doesn't). So if a user references the shaded
NettyServerBuilder without also depending on grpc-core directly,
compilation will fail because AbstractServerImplBuilder couldn't
be found.

This isn't technically a problem, since we're not wanting to encourage
users to reference the shaded classes directly. But some users will
certainly reference the classes anyway and the error is pretty confusing
while also being trivially worked around. In other words: it justs
wastes people's time and benefits nobody.

Fixes #5881

Prior to 745aa0a, this target was
visible publicly.

The 3rd-party rules_proto project is referencing our compiler directly
and not using our java_grpc_library. This target is fine for them to
use (although we'd prefer using our java_grpc_library), but most users
shouldn't be touching it.

Related to #5942 and #5947