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but has now been replaced by internal/fileinit.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/fileinit/desc_lazy.go b/internal/fileinit/desc_lazy.go
index 2f7d3ca..0ff208a 100644
--- a/internal/fileinit/desc_lazy.go
+++ b/internal/fileinit/desc_lazy.go
@@ -13,9 +13,9 @@
 	defval "github.com/golang/protobuf/v2/internal/encoding/defval"
 	wire "github.com/golang/protobuf/v2/internal/encoding/wire"
 	pimpl "github.com/golang/protobuf/v2/internal/impl"
+	ptype "github.com/golang/protobuf/v2/internal/prototype"
 	pvalue "github.com/golang/protobuf/v2/internal/value"
 	pref "github.com/golang/protobuf/v2/reflect/protoreflect"
-	ptype "github.com/golang/protobuf/v2/reflect/prototype"
 )
 
 func (file *fileDesc) lazyInit() *fileLazy {