Cherry-pick upstream commit to ensure that completions for commands
like mkfs.xfs(8) end up in mkfs.xfs.fish by fixing over-eager stripping
of file extensions.
All mkfs.*(8) (and fsck.*(8), etc.) programs would yield mkfs.fish files
with unique `complete -c mkfs ...` lines using the same command.
Thus completing `mkfs` in fish would for example yield XFS specific
suggestions, given "xfsprogs" was installed **and** won over other
packages.
Noticed through build logs:
```
building '/nix/store/5fz5jgkd2ycx5fk3lb30my2hr2br639f-system_fish-completions.drv'...
warning: collision between `/nix/store/1z01j2pma0l005w3qg6slndn0rp136s8-dosfstools-4.2_fish-completions/mkfs.fish' and `/nix/store/jxxgrng41l6qakg45fksk1qpil976vgi-xfsprogs-5.16.0_fish-completions/mkfs.fish'
```
The repository of the project has been transferred to a GitHub organisation.
See https://github.com/pure-fish/pure/issues/249.
The tests of this package are also failing upstream, so they have been
temporarily disabled here too.
Python is still used for tests, but does not become part of the closure.
In addition, nowadays fish only ever uses Python via __fish_anypython,
so using sed or propagating python isn't necessary anymore.
Replacing all occurrences of `/usr` with `/nix` doesn't work, because
one test is just verifying if `wdirname` works (and `dirname /usr/lib`
is not the same as `dirname /nix/lib`).
mkDerivation does not care about checkPlugins and checkFunctionDirs so let’s avoid polluting the scope.
Also remove installPath argument altogether for the same reason since it is not used at all.
