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/cmd/generate-protos/main.go b/internal/cmd/generate-protos/main.go
index 1763a92..3ab6c30 100644
--- a/internal/cmd/generate-protos/main.go
+++ b/internal/cmd/generate-protos/main.go
@@ -21,8 +21,8 @@
 
 	gengogrpc "google.golang.org/protobuf/cmd/protoc-gen-go-grpc/internal_gengogrpc"
 	gengo "google.golang.org/protobuf/cmd/protoc-gen-go/internal_gengo"
+	"google.golang.org/protobuf/compiler/protogen"
 	"google.golang.org/protobuf/internal/detrand"
-	"google.golang.org/protobuf/protogen"
 	"google.golang.org/protobuf/reflect/protoreflect"
 )