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/proto/decode_test.go b/proto/decode_test.go
index c1810c5..47dfdb4 100644
--- a/proto/decode_test.go
+++ b/proto/decode_test.go
@@ -10,7 +10,7 @@
 	"testing"
 
 	protoV1 "github.com/golang/protobuf/proto"
-	"google.golang.org/protobuf/encoding/textpb"
+	"google.golang.org/protobuf/encoding/prototext"
 	"google.golang.org/protobuf/internal/encoding/pack"
 	"google.golang.org/protobuf/internal/errors"
 	"google.golang.org/protobuf/internal/scalar"
@@ -1271,7 +1271,7 @@
 }
 
 func marshalText(m proto.Message) string {
-	b, _ := textpb.Marshal(m)
+	b, _ := prototext.Marshal(m)
 	return string(b)
 }