Fix loading namespaced doc_fragments from collections (#55249)

* 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

test/integration/targets/collections/collection_root_user/ansible_collections/testns/testcoll/plugins/doc_fragments/frag.py

@@ -0,0 +1,14 @@
+class ModuleDocFragment(object):
+    DOCUMENTATION = r'''
+options:
+  normal_doc_frag:
+    description:
+    - an option
+'''
+
+    OTHER_DOCUMENTATION = r'''
+options:
+  other_doc_frag:
+    description:
+    - another option
+'''

test/integration/targets/collections/collection_root_user/ansible_collections/testns/testcoll/plugins/modules/testmodule.py

@@ -2,6 +2,14 @@
 
 import json
 
+DOCUMENTATION = r'''
+module: testmodule
+description: for testing
+extends_documentation_fragment:
+  - testns.testcoll.frag
+  - testns.testcoll.frag.other_documentation
+'''
+
 
 def main():
     print(json.dumps(dict(changed=False, source='user')))

test/integration/targets/collections/runme.sh

@@ -20,6 +20,9 @@ fi
 # test callback
 ANSIBLE_CALLBACK_WHITELIST=testns.testcoll.usercallback ansible localhost -m ping | grep "usercallback says ok"
 
+# test documentation
+ansible-doc testns.testcoll.testmodule -vvv | grep -- "- normal_doc_frag"
+
 # we need multiple plays, and conditional import_playbook is noisy and causes problems, so choose here which one to use...
 if [[ ${INVENTORY_PATH} == *.winrm ]]; then
   export TEST_PLAYBOOK=windows.yml