internal/filedesc, internal/filetype: initial commit

The internal/fileinit package is split apart into two packages:
* internal/filedesc constructs descriptors from the raw proto.
It is very similar to the previous internal/fileinit package.
* internal/filetype wraps descriptors with Go type information

Overview:
* The internal/fileinit package will be deleted in a future CL.
It is kept around since the v1 repo currently depends on it.
* The internal/prototype package is deleted. All former usages of it
are now using internal/filedesc instead. Most significantly,
the reflect/protodesc package was almost entirely re-written.
* The internal/impl package drops support for messages that do not
have a Descriptor method (pre-2016). This removes a significant amount
of technical debt.
filedesc.Builder to parse raw descriptors.
* The internal/encoding/defval package now handles enum values by name.

Change-Id: I3957bcc8588a70470fd6c7de1122216b80615ab7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/182360
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/testprotos/test/ext.pb.go b/internal/testprotos/test/ext.pb.go
index 58cb656..7a3363c 100644
--- a/internal/testprotos/test/ext.pb.go
+++ b/internal/testprotos/test/ext.pb.go
@@ -5,7 +5,6 @@
 
 import (
 	protoreflect "google.golang.org/protobuf/reflect/protoreflect"
-	protoregistry "google.golang.org/protobuf/reflect/protoregistry"
 	protoiface "google.golang.org/protobuf/runtime/protoiface"
 	protoimpl "google.golang.org/protobuf/runtime/protoimpl"
 	sync "sync"
@@ -67,6 +66,11 @@
 }
 var file_test_ext_proto_depIdxs = []int32{
 	0, // goproto.proto.test.foreign_int32_extension:extendee -> goproto.proto.test.TestAllExtensions
+	1, // starting offset of method output_type sub-list
+	1, // starting offset of method input_type sub-list
+	1, // starting offset of extension type_name sub-list
+	0, // starting offset of extension extendee sub-list
+	0, // starting offset of field type_name sub-list
 }
 
 func init() { file_test_ext_proto_init() }
@@ -75,16 +79,19 @@
 		return
 	}
 	file_test_test_proto_init()
-	extensionTypes := make([]protoreflect.ExtensionType, 1)
-	File_test_ext_proto = protoimpl.FileBuilder{
-		RawDescriptor:        file_test_ext_proto_rawDesc,
-		GoTypes:              file_test_ext_proto_goTypes,
-		DependencyIndexes:    file_test_ext_proto_depIdxs,
-		LegacyExtensions:     file_test_ext_proto_extDescs,
-		ExtensionOutputTypes: extensionTypes,
-		FilesRegistry:        protoregistry.GlobalFiles,
-		TypesRegistry:        protoregistry.GlobalTypes,
-	}.Init()
+	out := protoimpl.TypeBuilder{
+		File: protoimpl.DescBuilder{
+			RawDescriptor: file_test_ext_proto_rawDesc,
+			NumEnums:      0,
+			NumMessages:   0,
+			NumExtensions: 1,
+			NumServices:   0,
+		},
+		GoTypes:           file_test_ext_proto_goTypes,
+		DependencyIndexes: file_test_ext_proto_depIdxs,
+		LegacyExtensions:  file_test_ext_proto_extDescs,
+	}.Build()
+	File_test_ext_proto = out.File
 	file_test_ext_proto_rawDesc = nil
 	file_test_ext_proto_goTypes = nil
 	file_test_ext_proto_depIdxs = nil