protogen: use protoreflect descriptors

Change the protogen types wrapping FileDescriptorProto et al. to use
protoreflect descriptors instead.

Change-Id: I99fe83b995a0a6f4fc88f03a6e4b827109a2ec80
Reviewed-on: https://go-review.googlesource.com/133815
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/cmd/protoc-gen-go/main.go b/cmd/protoc-gen-go/main.go
index e144aa6..bfae8eb 100644
--- a/cmd/protoc-gen-go/main.go
+++ b/cmd/protoc-gen-go/main.go
@@ -25,9 +25,9 @@
 }
 
 func genFile(gen *protogen.Plugin, f *protogen.File) {
-	g := gen.NewGeneratedFile(strings.TrimSuffix(f.Desc.GetName(), ".proto")+".pb.go", f.GoImportPath)
+	g := gen.NewGeneratedFile(strings.TrimSuffix(f.Desc.Path(), ".proto")+".pb.go", f.GoImportPath)
 	g.P("// Code generated by protoc-gen-go. DO NOT EDIT.")
-	g.P("// source: ", f.Desc.GetName())
+	g.P("// source: ", f.Desc.Path())
 	g.P()
 	g.P("package TODO")
 	g.P()