There were a load of warnings such as the below, when building the
protobufs:
```
warning: Import google/protobuf/empty.proto is unused.
```
This removes the unused imports.
The codegen and test files changed because the old sed script wasn't
correctly fixing up those files, see
https://github.com/pulumi/pulumi/pull/14424 for more on attempting to
fix those scripts. But this gets rid of one of those sed scripts
entirely.
This changes codegen to be invoked via gRPC from pkg, rather than
invoking pkg/codegen directly.
Consider it a proof-of-concept for moving codegen to a gRPC interface
without the worries of forwards-backwards compatability (because we ship
language plugins at a fixed version side-by-side to users).