Mark some Bazel targets as "manual." (#7932)
Mark some Bazel targets as "manual." (#7932)

The Python C++ extension targets are not used unless
`--@:use_fast_cpp_protos=true`, and may not even be able to build
if the Python headers are missing (note that
`//util/python:python_headers`, bound to `@python_headers//` in
`//:WORKSPACE`, is not currently sufficient). This change adds the
`"manual"` tag to these targets, so that they do not cause
`bazel test ...` to fail when Python headers are missing. Without
the manual tag, the targets are always selected, even if
`--@:use_fast_cpp_protos=false`.

The `:cc_proto_blacklist_test` target is metastable, depending on
whether the `--proto_toolchain_for_cc=` flag names a target with
or without the `@com_google_protobuf//` prefix. We use the correct
prefix for Bazel's default in `kokoro/linux/bazel/build.sh`, but
the `bazel test :cc_proto_blacklist_test` (with or without `//`)
fails consistently. Hopefully, this will be fixed when
bazelbuild/bazel#10590 is addressed.
google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf