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but has now been replaced by internal/fileinit.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/cmd/generate-types/main.go b/internal/cmd/generate-types/main.go
index 8fa3222..0c0a34d 100644
--- a/internal/cmd/generate-types/main.go
+++ b/internal/cmd/generate-types/main.go
@@ -40,7 +40,7 @@
 	chdirRoot()
 	writeSource("internal/fileinit/desc_list_gen.go", generateFileinitDescList())
 	writeSource("proto/decode_gen.go", generateProtoDecode())
-	writeSource("reflect/prototype/protofile_list_gen.go", generateListTypes())
+	writeSource("internal/prototype/protofile_list_gen.go", generateListTypes())
 }
 
 // chdirRoot changes the working directory to the repository root.
diff --git a/internal/cmd/pbdump/pbdump.go b/internal/cmd/pbdump/pbdump.go
index fc96696..c6cb7c7 100644
--- a/internal/cmd/pbdump/pbdump.go
+++ b/internal/cmd/pbdump/pbdump.go
@@ -19,9 +19,9 @@
 
 	"github.com/golang/protobuf/v2/internal/encoding/pack"
 	"github.com/golang/protobuf/v2/internal/encoding/wire"
+	"github.com/golang/protobuf/v2/internal/prototype"
 	"github.com/golang/protobuf/v2/internal/scalar"
 	"github.com/golang/protobuf/v2/reflect/protoreflect"
-	"github.com/golang/protobuf/v2/reflect/prototype"
 
 	descriptorpb "github.com/golang/protobuf/v2/types/descriptor"
 )
diff --git a/internal/cmd/pbdump/pbdump_test.go b/internal/cmd/pbdump/pbdump_test.go
index ddc20a1..9e50ec4 100644
--- a/internal/cmd/pbdump/pbdump_test.go
+++ b/internal/cmd/pbdump/pbdump_test.go
@@ -12,8 +12,8 @@
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 
+	ptype "github.com/golang/protobuf/v2/internal/prototype"
 	pref "github.com/golang/protobuf/v2/reflect/protoreflect"
-	ptype "github.com/golang/protobuf/v2/reflect/prototype"
 )
 
 func TestFields(t *testing.T) {