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/testing/prototest/prototest.go b/testing/prototest/prototest.go
index c9f7c7a..c24a811 100644
--- a/testing/prototest/prototest.go
+++ b/testing/prototest/prototest.go
@@ -12,7 +12,7 @@
 	"sort"
 	"testing"
 
-	textpb "google.golang.org/protobuf/encoding/textpb"
+	prototext "google.golang.org/protobuf/encoding/prototext"
 	"google.golang.org/protobuf/proto"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
 )
@@ -83,7 +83,7 @@
 }
 
 func marshalText(m proto.Message) string {
-	b, _ := textpb.MarshalOptions{Indent: "  "}.Marshal(m)
+	b, _ := prototext.MarshalOptions{Indent: "  "}.Marshal(m)
 	return string(b)
 }
 
@@ -346,7 +346,7 @@
 		buf.WriteString("]")
 		return buf.String()
 	case pref.Message:
-		b, err := textpb.Marshal(v.Interface())
+		b, err := prototext.Marshal(v.Interface())
 		if err != nil {
 			return fmt.Sprintf("<%v>", err)
 		}