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/impl/legacy_file.go b/internal/impl/legacy_file.go
index ddf33f7..b1293c6 100644
--- a/internal/impl/legacy_file.go
+++ b/internal/impl/legacy_file.go
@@ -9,6 +9,10 @@
 	"compress/gzip"
 	"io/ioutil"
 	"sync"
+
+	"google.golang.org/protobuf/internal/filedesc"
+	"google.golang.org/protobuf/reflect/protoreflect"
+	"google.golang.org/protobuf/reflect/protoregistry"
 )
 
 // Every enum and message type generated by protoc-gen-go since commit 2fc053c5
@@ -28,7 +32,7 @@
 	}
 )
 
-var legacyFileDescCache sync.Map // map[*byte]*descriptorpb.FileDescriptorProto
+var legacyFileDescCache sync.Map // map[*byte]protoreflect.FileDescriptor
 
 // legacyLoadFileDesc unmarshals b as a compressed FileDescriptorProto message.
 //
@@ -36,10 +40,10 @@
 // concatenated series of GZIP files (which would require shenanigans that
 // rely on the concatenation properties of both protobufs and GZIP).
 // File descriptors generated by protoc-gen-go do not rely on that property.
-func legacyLoadFileDesc(b []byte) *legacyFileDescriptorProto {
+func legacyLoadFileDesc(b []byte) protoreflect.FileDescriptor {
 	// Fast-path: check whether we already have a cached file descriptor.
 	if fd, ok := legacyFileDescCache.Load(&b[0]); ok {
-		return fd.(*legacyFileDescriptorProto)
+		return fd.(protoreflect.FileDescriptor)
 	}
 
 	// Slow-path: decompress and unmarshal the file descriptor proto.
@@ -47,13 +51,23 @@
 	if err != nil {
 		panic(err)
 	}
-	b, err = ioutil.ReadAll(zr)
+	b2, err := ioutil.ReadAll(zr)
 	if err != nil {
 		panic(err)
 	}
-	fd := legacyParseFileDescProto(b)
+
+	fd := filedesc.DescBuilder{
+		RawDescriptor: b2,
+		FileRegistry:  resolverOnly{protoregistry.GlobalFiles}, // do not register back to global registry
+	}.Build().File
 	if fd, ok := legacyFileDescCache.LoadOrStore(&b[0], fd); ok {
-		return fd.(*legacyFileDescriptorProto)
+		return fd.(protoreflect.FileDescriptor)
 	}
 	return fd
 }
+
+type resolverOnly struct {
+	*protoregistry.Files
+}
+
+func (resolverOnly) Register(...protoreflect.FileDescriptor) error { return nil }