Consider plugins whose last component of FQCN starts with '_' as private

[felixfontein]

Implementation for ansible-community/community-topics#154

Once this is merged plugins outside ansible.builtin whose last FQCN component starts with _ are treated as private and marked as such in their documentation, and are no longer listed in plugin indexes.

As an example see the following links, which refer to the hidden filter plugin community.sops._latest_version from ansible-collections/community.sops#98:

• https://ansible.fontein.de/collections/community/sops/#filter-plugins does not show the filter
• https://ansible.fontein.de/collections/community/sops/_latest_version_filter.html shows the filter's documentation, with a PRIVATE notice (https://ansible.fontein.de/collections/community/sops/_latest_version_filter.html#private).