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/encoding/jsonpb/jsonpb.go b/encoding/jsonpb/jsonpb.go
new file mode 100644
index 0000000..43c28b5
--- /dev/null
+++ b/encoding/jsonpb/jsonpb.go
@@ -0,0 +1,14 @@
+// Package jsonpb is deprecated.
+package jsonpb
+
+import "google.golang.org/protobuf/encoding/protojson"
+
+var (
+	Marshal   = protojson.Marshal
+	Unmarshal = protojson.Unmarshal
+)
+
+type (
+	MarshalOptions   = protojson.MarshalOptions
+	UnmarshalOptions = protojson.UnmarshalOptions
+)