Multi protobuf module emission and consumption (backport #2344) (#2352)

* Multi protobuf module emission and consumption (#2344)

* Add compiler option (`-p`) to emit individual module protobufs
* Implement multi module combination when reading directory of protobufs

Co-authored-by: Jack Koenig <koenig@sifive.com>
(cherry picked from commit 0c1ca581

)

* Multi PB backport fixes (#2353)

Co-authored-by: Jared Barocsi <82000041+jared-barocsi@users.noreply.github.com>