Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stubs/backports/METADATA.toml

@@ -0,0 +1,2 @@
+version = "0.1"
+python2 = true

stubs/backports/backports/__init__.pyi

@@ -0,0 +1,4 @@
+from typing import Any
+
+# Explicitly mark this package as incomplete.
+def __getattr__(name: str) -> Any: ...

stubs/backports/backports/ssl_match_hostname.pyi

@@ -0,0 +1,3 @@
+class CertificateError(ValueError): ...
+
+def match_hostname(cert, hostname): ...