internal/prototype: move from reflect/prototype

The prototype package was initially used by generated reflection support,
but has now been replaced by internal/fileinit.
Eventually, this functionality should be deleted and re-written in terms
of other components in the repo.

Usages that prototype currently provides (but should be moved) are:
* Constructing standalone messages and enums, which is behavior we should
provide in reflect/protodesc. The google.protobuf.{Enum,Type} are well-known
proto messages designed for this purpose.
* Constructing placeholder files, enums, and messages.
* Consructing protoreflect.{Message,Enum,Extension}Types, which are protobuf
descriptors with associated Go type information.

Change-Id: Id7dbefff952682781b439aa555508c59b2629f9e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/167383
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/cmd/protoc-gen-go/internal_gengo/reflect.go b/cmd/protoc-gen-go/internal_gengo/reflect.go
index e11dc15..55fb42c 100644
--- a/cmd/protoc-gen-go/internal_gengo/reflect.go
+++ b/cmd/protoc-gen-go/internal_gengo/reflect.go
@@ -27,7 +27,7 @@
 	reflectPackage      = protogen.GoImportPath("reflect")
 	protoimplPackage    = protogen.GoImportPath("github.com/golang/protobuf/v2/runtime/protoimpl")
 	protoreflectPackage = protogen.GoImportPath("github.com/golang/protobuf/v2/reflect/protoreflect")
-	prototypePackage    = protogen.GoImportPath("github.com/golang/protobuf/v2/reflect/prototype")
+	prototypePackage    = protogen.GoImportPath("github.com/golang/protobuf/v2/internal/prototype")
 )
 
 // TODO: Add support for proto options.