Audit `additional_proto_files`

LROs have an annotation for their returned metadata type. Proto syntax does not require `import`s for the proto files that define these metadata types. In C++ we have to include these files.

Our intention for `additional_proto_files` was to add includes for these extra `*.pb.h` headers.

Turns out that when we feed them into `protoc` we treat them the same as any other service's proto file.

https://github.com/googleapis/google-cloud-cpp/blob/a86e887fe2756935a51d4d93b42e45a0fdde7812/generator/standalone_main.cc#L314-L317

This leads to some weirdness in the generator config. In `notebooks` and `run`, we rely on this behavior to generate some clients (and avoid #7997). In `containeranalysis`, we accidentally generate a client for `grafeas` which we did not intend to. Oops.

We should sort this out.