Include generated files in releases of packages depending on proto-lens

[judah]

We should do something similar to happy/alex/etc for generated files, i.e., bundle them into the release archive that's uploaded to Hackage/Stackage. That way, packages that depend on protos won't require installing the protoc executable.

Cabal has special logic for happy and alex, but the logic around when to rebuild the generated files is somewhat flaky: haskell/cabal#2940, haskell/cabal#2311, haskell/cabal#2362. Part of the problem is that when Cabal unpacks the tarball of the package, it doesn't set the modification times consistently (this may be fixed on newer versions of Cabal, not sure though).

One option is for us to do something simpler than Cabal:

• Never run protoc when building from an archive that was created by cabal sdist
• Always run protoc otherwise (in particular: when building from the git repo).
  We'd need to make cabal sdist do something special in order for cabal build to tell the difference. One hacky option is to include an extra dummy file in extra-src-files. A more involved option would be to copy the generated files from the autogen dir (where they are now) to one of the hs-source-dirs; but that may be complicated in the presence of multiple binaries/tests.

[judah]

I'm closing this ticket since we've continued to use the .proto files as the source of truth, and don't have plans to change that. Additionally, Cabal now supports an autogen-modules field which makes it more aware of these generated modules and allowed us to remove the custom sdist hook. I'd rather keep it that way than add back more complexity to proto-lens-setup.