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/reflect/protodesc/protodesc_test.go b/reflect/protodesc/protodesc_test.go
index 19c3ab3..1ef9c43 100644
--- a/reflect/protodesc/protodesc_test.go
+++ b/reflect/protodesc/protodesc_test.go
@@ -1,3 +1,7 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package protodesc
 
 import (
@@ -439,7 +443,7 @@
 					Name:     scalar.String("nested_message"),
 					Number:   scalar.Int32(4),
 					Label:    descriptorpb.FieldDescriptorProto_LABEL_OPTIONAL.Enum(),
-					Type:     descriptorpb.FieldDescriptorProto_TYPE_ENUM.Enum(),
+					Type:     descriptorpb.FieldDescriptorProto_TYPE_MESSAGE.Enum(),
 					TypeName: scalar.String(".foo.TopLevelMessage.NestedMessage"),
 				}},
 			}},
@@ -596,14 +600,14 @@
 			for _, dep := range tc.deps {
 				f, err := NewFile(dep, r)
 				if err != nil {
-					t.Fatalf("Error creating dependency: %v", err)
+					t.Fatalf("error creating dependency: %v", err)
 				}
 				if err := r.Register(f); err != nil {
-					t.Fatalf("Error adding dependency: %v", err)
+					t.Fatalf("error adding dependency: %v", err)
 				}
 			}
 			if _, err := NewFile(tc.fd, r); err != nil {
-				t.Errorf("NewFile: got err = %v", err)
+				t.Errorf("unexpected NewFile error: %v", err)
 			}
 		})
 	}