* Fix loading namespaced doc_fragments
The syntax for specifying a different fragment name was already
using '.' as a separator, so the code needed to be tweaked to
avoid choking on names like `testns.testcoll.fragname` and
`testns.testcoll.fragname.altvar`.
`get_plugin_class()` returns 'docfragment' for the fragment loader;
mangling `subdir` provides consistent alignment with the normal plugin
directory names and avoids needing special handling of plugin types
with 'module' in the name.
* Add changelog entry
