this is based on the xce-unfederated branch of the workadventure-xce
repository [1], and requires some reworking of the general derivation
code since it currently uses older package sets than the other versions.

[1] https://github.com/workadventure-xce/workadventure-xce/tree/xce-unfederated

wapkgs.nix new contains packages for several different versions of
workadventure (so far hasi + thecodingmachine), which can both be used.
Unfortunately, this appears to require some weird hacks to convince nix
to sort out the dependencies without having to actually duplicate all
the packaging code, but it works.

Maximal sharing means that even if the workadventure sources are imported
in all nix files, there will still only be one copy of it in the nix store
--- nevertheless, it seems reasonable to only have them once in the code,
too, if only to make upgrading them easier.

Builds fine, let's see if anything else works!