Renames files from selector to select

Since the `Selector` class is gone, it makes sense to name files as
`select` again.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/channels/util/__init__.py

@@ -31,7 +31,7 @@
 from ._file_watcher import FileWatcher
 from ._merge import Merge
 from ._merge_named import MergeNamed
-from ._selector import (
+from ._select import (
     Selected,
     SelectError,
     SelectErrorGroup,