internal/impl: make resolverOnly explicitly implement file resolver

The resolverOnly type embeds a *protoregistry.Files and overrides one of
of its methods. When that method was renamed (Register -> RegisterFile),
the resolverOnly type continued to compile but no longer overrode the
desired method.

Update the method name, and change this type to explicitly forward
methods to the underlying *protoregistry.Files to catch errors of this
nature in the future.

Change-Id: I09a529034b6e2f310977bc635288fa15ce14cc7e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/205242
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/internal/impl/legacy_file.go b/internal/impl/legacy_file.go
index b61a135..9ab0910 100644
--- a/internal/impl/legacy_file.go
+++ b/internal/impl/legacy_file.go
@@ -67,7 +67,15 @@
 }
 
 type resolverOnly struct {
-	*protoregistry.Files
+	reg *protoregistry.Files
 }
 
-func (resolverOnly) Register(protoreflect.FileDescriptor) error { return nil }
+func (r resolverOnly) FindFileByPath(path string) (protoreflect.FileDescriptor, error) {
+	return r.reg.FindFileByPath(path)
+}
+func (r resolverOnly) FindDescriptorByName(name protoreflect.FullName) (protoreflect.Descriptor, error) {
+	return r.reg.FindDescriptorByName(name)
+}
+func (resolverOnly) RegisterFile(protoreflect.FileDescriptor) error {
+	return nil
+}