WIP: replaced assets-misc/atl-* with assets-misc/project-atlas

I think this takes the title of the most complex mod in the repo, though Morrowind beats it in terms of sheer size due to its whitelist for fallback values.

This is primarily due to the atlas generating code. Not only do we need to collect all of the required files from the VFS (also done for tes3cmd's plugin cleaning and isn't that complicated on its own), we also have to get the windows bat scripts running on other platforms. This is done with the use of an extremely tailored python script that uses mostly regexes and a small amount of context-sensitive code. This will likely need to be checked and updated when project atlas updates, but hopefully it will require less maintenance than manual conversion of the scripts.

Update: I've added support for normal and specular maps via a MAP use_expand. Unfortunately I don't think we have mods in the tree that currently satisfy the required files. This is certainly a place where portmod#52 (closed) would be really handy, as we could flag mods as satisfying the asset groups required by project-atlas, and have those be a requirement for enabling specular/normal maps (we also need the VFS to support BSA extraction at some point so that this mod doesn't fail if there aren't mods covering the textures installed).

TODO:

• New USE_EXPAND flags are causing issues, despite working locally for me and the CI reporting that it has 2.0a5 installed.
• _unpack_sequence_ needs to be added to the sandbox (a guarded version is included in RestrictedPython).
• Either have the atlas generation be optional, or add support for extracting files from bsas to Portmod.

closes #21 (closed) #30 (closed) #31 (closed) #32 (closed) #33 (closed) #36 (closed) #37 (closed) #38 (closed) #39 (closed)

Also includes !88 (closed), as it uses the new AtlasGen class