Fix imports and general cleanup

· Import from compat what comes from compat. Yes, some names are available in utils too, but that's an implementation detail.
· Use _match_id consistently whenever possible
· Fix some outdated tests
· Use consistent valid URL (always match the whole protocol, no ^ at start required)
· Use modern test definitions

youtube_dl/downloader/common.py

@@ -5,8 +5,8 @@ import re
 import sys
 import time
 
+from ..compat import compat_str
 from ..utils import (
-    compat_str,
     encodeFilename,
     format_bytes,
     timeconvert,