Sysroot rule creates inescapable warning cycle

[phlax]

Since #572 landed the toolchain warns if you use a globbed filegroup for sysroot

If you follow the suggestion and use the sysroot rule then it creates many more warnings - like:

WARNING: /source/test/test_common/BUILD:196:22: input 'sysroot' of //test/test_common:file_system_for_test_lib is a directory; dependency checking of directories is unsound
WARNING: /source/source/common/network/BUILD:180:17: input 'sysroot' of //source/common/network:filter_manager_lib is a directory; dependency checking of directories is unsound
WARNING: /source/source/common/api/BUILD:12:17: input 'sysroot' of //source/common/api:api_lib is a directory; dependency checking of directories is unsound
WARNING: /source/source/common/event/BUILD:15:17: input 'sysroot' of //source/common/event:dispatcher_lib is a directory; dependency checking of directories is unsound
WARNING: /source/source/common/event/BUILD:15:17: input 'sysroot' of //source/common/event:dispatcher_lib is a directory; dependency checking of directories is unsound
WARNING: /source/test/BUILD:35:22: input 'sysroot' of //test:main is a directory; dependency checking of directories is unsound
WARNING: /source/test/BUILD:35:22: input 'sysroot' of //test:main is a directory; dependency checking of directories is unsound
WARNING: /source/test/test_common/BUILD:124:22: input 'sysroot' of //test/test_common:utility_lib is a directory; dependency checking of directories is unsound
WARNING: /source/test/test_common/BUILD:16:22: input 'sysroot' of //test/test_common:environment_lib is a directory; dependency checking of directories is unsound
WARNING: /source/source/common/network/BUILD:152:17: input 'sysroot' of //source/common/network:default_client_connection_factory is a directory; dependency checking of directories is unsound

i think that #572 is incorrect in bazel terms - those files should be tracked, and its warning also incorrect

[fmeum]

You can use the --host_jvm_args=-DBAZEL_TRACK_SOURCE_DIRECTORIES=1 startup flag to resolve the problem.

@dzbarsky Could you send a PR to gate this feature behind a bazel_features feature for incrementally correct source directory tracking by default?