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/impl/export.go b/internal/impl/export.go
index ae0e9fa..9ea3900 100644
--- a/internal/impl/export.go
+++ b/internal/impl/export.go
@@ -8,7 +8,7 @@
 	"reflect"
 	"strconv"
 
-	"google.golang.org/protobuf/encoding/textpb"
+	"google.golang.org/protobuf/encoding/prototext"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
 )
 
@@ -111,6 +111,6 @@
 // MessageStringOf returns the message value as a string,
 // which is the message serialized in the protobuf text format.
 func (Export) MessageStringOf(m pref.ProtoMessage) string {
-	b, _ := textpb.MarshalOptions{AllowPartial: true}.Marshal(m)
+	b, _ := prototext.MarshalOptions{AllowPartial: true}.Marshal(m)
 	return string(b)
 }