* add subdir support to collection loading
* collections may now load plugins from subdirs under a plugin type or roles dir, eg `ns.coll.subdir1.subdir2.myrole`->ns.coll's roles/subdir1/subdir2/myrole, `ns.coll.subdir1.mymodule`->ns.coll's plugins/modules/subdir1/mymodule.py
* centralize parsing/validation in AnsibleCollectionRef class
* fix issues loading Jinja2 plugins from multiple sources
* resolves#59462, #59890,
* sanity test fixes
* string fixes
* add changelog entry
