backends: make NewObject return fs.ErrorIsDir if possible

This changes the interface to NewObject so that if NewObject is called
on a directory then it should return fs.ErrorIsDir if possible without
doing any extra work, otherwise fs.ErrorObjectNotFound.

Tested on integration test server with:

go run integration-test.go -tests backend -run TestIntegration/FsMkdir/FsPutFiles/FsNewObjectDir -branch fix-stat -maxtries 1

backend/sharefile/sharefile.go

@@ -337,8 +337,12 @@ func (f *Fs) readMetaDataForIDPath(ctx context.Context, id, path string, directo
 	if directoriesOnly && item.Type != api.ItemTypeFolder {
 		return nil, fs.ErrorIsFile
 	}
-	if filesOnly && item.Type != api.ItemTypeFile {
-		return nil, fs.ErrorNotAFile
+	if filesOnly {
+		if item.Type == api.ItemTypeFolder {
+			return nil, fs.ErrorIsDir
+		} else if item.Type != api.ItemTypeFile {
+			return nil, fs.ErrorNotAFile
+		}
 	}
 	return &item, nil
 }