support previous root collections & roles.

[svennd]

This would allow the previous behaviour aswell as the new behaviour : (issue #2519)

Currently allowed :

<playbook_dir>/collections/requirements.yml
<playbook_dir>/roles/requirements.yml

suggested change :

<playbook_dir>/collections/requirements.yml
<playbook_dir>/roles/requirements.yml
<repo_dir>/collections/requirements.yml
<repo_dir>/roles/requirements.yml

This makes it possible to utilise subdir's to put the playbooks in, sharing a common role or collection. Eg :

/collections/requirements.yml
/debug/play_tester.yml
/maintain/play_maintenance_a.yml
/maintain/play_maintenance_b.yml

this makes more sense to maintain then :

/debug/collections/requirements.yml
/debug/play_tester.yml

/maintain/collections/requirements.yml
/maintain/play_maintenance_a.yml
/maintain/play_maintenance_b.yml

A more proper way might be possible!