Test collection hook to ignore uninstallable addons

[sbidoul]

This is useful to use pytest (and IDE test discovery) in migration projects where we have a bunch of uninstallable addons.

[sbidoul]

@baimont @rousseldenis @simahawk

[sbidoul]

@simahawk this should now be ready.

I took your suggestion to search for manifests more aggressively.

I also added here two commits to make pytest-odoo behave better when running non-odoo tests in an environment where pytest-odoo is installed.