Render package __init__.py at the directory's index.html

[danceratopz]

Summary

• Route __init__.py sources to <dir>/index so each package's landing page (index.html) is the rendered module documentation instead of an empty directory listing
• Keep the synthetic ListingSource in the navigation tree, but skip writing its file in cli.py when a non-listing source already covers the same output path — this preserves the docc/, plugins/, etc. entries in parent listings while letting the module page win at index.html
• The module template already renders a leaf siblings listing under "Browse", so the directory listing remains visible on the package's index page

Example

Test plan

• tox -e py3 passes (lint + tests + docc self-build)
• tox -e type passes (pyre)
• Generated docs no longer contain any __init__.py.html files; index.html exists for every package and contains the rendered __init__.py
• Cross-references and breadcrumbs resolve to <pkg>/index.html
• Parent listings still link to subpackages (e.g. src/index.html lists docc/)

[SamWilsn]

This is giving special privilege to the listing plugin, and feels a bit hacky.

I'm not sure what a better solution in this vein would be. Maybe every source could have a priority and the highest priority claims the output path? That way there's no special case for listings.

That said, I think the better solution is still to not generate the listing in the first place, but I've been having some difficulty coming up with an elegant implementation of that.