cooker: Further optimise pyinotify

cooker: Further optimise pyinotify

We currently add crazy numbers of watches on files. The per user limit is 8192
by default and on a system handling multiple builds, this can be an issue.

We don't need to watch all files individually, we can watch the directory containing
the file instead. This gives better resource utilisation and better performance
further reverting some of the performance regression seen with the introduction
of pyinotify.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>