Since protoc is not go-gettable, and most development tasks in Terraform
won't involve recompiling protoc files anyway, we'll use a separate
mechanism for these.

This way "go generate" only depends on things we can "go get" in the
"make tools" target.

In a later commit we should also in some way specify a particular version
of protoc to use so that we don't get "flapping" regenerations as
developers work with different versions, but the priority here is just to
make "make generate" minimally usable again to restore the dev workflow
documented in the README.

This also includes some updates that resulted from running "make generate"
and "make protobuf" after those Makefile changes were in place.