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but has now been replaced by internal/fileinit.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/internal/impl/message_test.go b/internal/impl/message_test.go
index b7f3770..71bcb70 100644
--- a/internal/impl/message_test.go
+++ b/internal/impl/message_test.go
@@ -13,10 +13,10 @@
 
 	protoV1 "github.com/golang/protobuf/proto"
 	pimpl "github.com/golang/protobuf/v2/internal/impl"
+	ptype "github.com/golang/protobuf/v2/internal/prototype"
 	scalar "github.com/golang/protobuf/v2/internal/scalar"
 	pvalue "github.com/golang/protobuf/v2/internal/value"
 	pref "github.com/golang/protobuf/v2/reflect/protoreflect"
-	ptype "github.com/golang/protobuf/v2/reflect/prototype"
 	cmp "github.com/google/go-cmp/cmp"
 	cmpopts "github.com/google/go-cmp/cmp/cmpopts"