* config: singular ANSIBLE_COLLECTIONS_PATH
Every other *_PATH setting in ansible is singular, and the traditional
$PATH variable is also singular despite containing a list of
directories. Let's be consistent both internally and with POSIX
tradition.
* update all ANSIBLE_COLLECTIONS_PATHS env references to be singular
* deprecate plural ANSIBLE_COLLECTIONS_PATHS setting
* find_module can't pop ImportError- we need to just translate to `None` since this is a normal condition with files on sys.path (eg `/usr/lib/python36.zip`)
* added test
* `meta/` directory in collections
* runtime metadata for redirection/deprecation/removal of plugin loads
* a compatibility layer to keep existing content working on ansible-base + collections
* a Python import redirection layer to keep collections-hosted (and otherwise moved) content importable by things that don't know better
* supported Ansible version validation on collection loads
