Delete buf protoc

[bufdev]

This PR effectively deletes buf protoc.

buf protoc was added to demonstrate to users that buf actually does produce equivalent compilation results to protoc, and to give them a mechanism to verify this. However, protoc is a poor experience for working with Protobuf - this poor experience is part of why buf exists. The experience is so sufficiently different (and difficult to isolate) that we have what amounts to a very different internal code path to setup the compilation. These differences result in a maintenance burden for something that we want to deprecate in the first place.

Issues are piling up that we don't have bandwidth to address anytime soon:

• Fix duplicate plugin execution for buf protoc #776
• buf protoc command line option inconsistency with protoc #491
• protoc not resolving arguments using include paths #353
• Relax root overlap requirements for buf protoc #351
• Add buf protoc --descriptor_set_in #357
• Feature parity between buf protoc and protoc #496

We want to spend our time fixing the Protobuf experience as a whole, not maintaining something that isn't a great experience in the first place. As such, we're removing buf protoc pre-v1.0.

What this PR actually does is moves buf protoc to buf alpha protoc. buf alpha is a hidden, completely experimental sub-command that has zero guarantees - in fact, we expect most of the commands under buf alpha to be deleted at some point. Having buf alpha protoc is useful for buf development at times as when we get a compilation issue, we can directly compare protoc and buf alpha protoc results to see if the issue is also present in protoc. If this usefulness wanes over time, and/or we have sufficient need to delete the divergent codepaths, we can delete it outright.

An alternative was to split this out into a separate buf-protoc or protoc-buf binary, that either lives here or in another repository, but this would seem to incur even more maintenance.

Few users are using buf protoc itself, and the mitigation is trivial: go back to using mainline protoc. buf protoc itself requires mainline protoc to be installed anyways if using any of the builtin plugins (--java_out, --cpp_out, etc).