Remove the generated proto packages that already exist in
google.golang.org/genproto. We want to eventually move these
packages here, but it doesn't need to happen yet.
Add a local copy of fieldmaskpb for use in tests.
Refactor proto generation to override import paths using the
M<source>=<import_path> compiler option instead of by patching the
source files.
Change-Id: I8d31f67e931d70140182f19f3e0106111f71c4b4
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/219598
Reviewed-by: Joe Tsai <joetsai@google.com>
