* fix coverage output from synthetic packages
* synthetic packages (eg, implicit collection packages without `__init__.py`) were always created at runtime with empty string source, which was compiled to a code object and exec'd during the package load. When run with code coverage, it created a bogus coverage entry (since the `__synthetic__`-suffixed `__file__` entry didn't exist on disk).
* modified collection loader `get_code` to preserve the distinction between `None` (eg synthetic package) and empty string (eg empty `__init__.py`) values from `get_source`, and to return `None` when the source is `None`. This allows the package loader to skip `exec`ing things that truly have no source file on disk, thus not creating bogus coverage entries, while preserving behavior and coverage reporting for empty package inits that actually exist.
* add unit test
* 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
