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/cmd/protoc-gen-go/internal_gengo/main.go b/cmd/protoc-gen-go/internal_gengo/main.go
index 801590b..37bdc3d 100644
--- a/cmd/protoc-gen-go/internal_gengo/main.go
+++ b/cmd/protoc-gen-go/internal_gengo/main.go
@@ -16,9 +16,9 @@
 	"unicode"
 	"unicode/utf8"
 
+	"google.golang.org/protobuf/compiler/protogen"
 	"google.golang.org/protobuf/internal/encoding/tag"
 	"google.golang.org/protobuf/internal/fieldnum"
-	"google.golang.org/protobuf/protogen"
 	"google.golang.org/protobuf/reflect/protoreflect"
 
 	descriptorpb "google.golang.org/protobuf/types/descriptor"
@@ -711,7 +711,7 @@
 	return "// Deprecated: Do not use."
 }
 
-// TODO: Remove this. This was added to aid jsonpb, but jsonpb does this work
+// TODO: Remove this. This was added to aid protojson, but protojson does this work
 // through the use of protobuf reflection now.
 func genWellKnownType(g *protogen.GeneratedFile, ptr string, ident protogen.GoIdent, desc protoreflect.Descriptor) {
 	if wellKnownTypes[desc.FullName()] {