protogen, encoding/jsonpb, encoding/textpb: rename packages

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>
diff --git a/internal/legacy/enum.go b/internal/legacy/enum.go
index 67a43f9..d750f1e 100644
--- a/internal/legacy/enum.go
+++ b/internal/legacy/enum.go
@@ -152,7 +152,7 @@
 		// golang/protobuf so the enum may not even be found in the registry.
 		//
 		// Instead, create a bogus enum descriptor to ensure that
-		// most operations continue to work. For example, textpb and jsonpb
+		// most operations continue to work. For example, prototext and protojson
 		// will be unable to parse a message with an enum value by name.
 		e.Syntax = pref.Proto2
 		e.FullName = deriveFullName(t)