java_grpc_library.bzl: Make warnings less noisy

[mkosiba]

This is to partially address #11791.
The idea is to only inform the user if one of labels are in the root repository and therefore under the user's control.

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[ejona86]

I'm not aware of any Bazel tooling that only reports warnings when it is under the user's control. Are you?

[mkosiba]

I'm not aware of any Bazel tooling that only reports warnings when it is under the user's control. Are you?

Plenty of tooling behaves this way:

• bazel coverage won't cover stuff in external/ (which is usually 3rd party code) by default,
• most linters and static code analysis behave this way (say, for Java your linters won't tell you about potential issues inside of the jars you're compiling against).

I can narrow this down to just grpc-java and grpc-proto if that's somehow better. Either way I think it's counter-productive to keep a warning that's "unfortunately expected" (as per referenced issue). At work at I've got internal users occasionally pinging me about this, trying to figure out if they're importing the wrong versions of deps or whatever, so this is affecting real users.