use proto's go_package option (#17)

* reimplement regen

* regenerate files

This change should fix our ongoing problem with duplicate file names causing failures.

At the moment, this changes requires you to build protobuf from source with this patch to work. Otherwise, we won't be able to properly import descriptor.proto.

- all proto files are deleted, as the new logic does not need to rewrite any part of the proto files.
- google.protobuf.descriptor is removed; it's official home is github.com/golang/protobuf/protoc-gen-go/descriptor.
- the protobuf directory is split into multiple packages. This is a breaking change.