Add utility to check if file is executable

This replaces direct calls to “os.access” and
“os.path.exists”/“os.path.isfile”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>