Fixes #600 and fixes #605

This revisits the buf generate command so that we consolidate all of our generation logic in appproto and appprotoos. With this, buf protoc, buf generate, and our server-side remote generation all use the same code paths.

This also fixes how we handle insertion points so that our behavior is aligned with protoc: we only permit insertion points to act upon other plugins configured to the same _out directory. For example,

$ protoc --insertion-point-receiver_out=gen/proto/insertion --insertion-point-writer_out=gen/proto/insertion

The insertion-point-writer will only have access to the files deposited in gen/proto/insertion, which is correctly configured above. Note that this was not previously the case.

Aside from the unit tests included here, this was tested against a variety of different cases, including combinations of remote and local plugins (including insertion points on either side).