Rename encoding/*pb to follow the convention of prefixing package names
with 'proto':
google.golang.org/protobuf/encoding/protojson
google.golang.org/protobuf/encoding/prototext
Move protogen under a compiler/ directory, just in case we ever do add
more compiler-related packages.
google.golang.org/protobuf/compiler/protogen
Change-Id: I31010cb5cabcea8274fffcac468477b58b56e8eb
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/177178
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>