bug#44662: Lua search paths

[raingloom <raingloom@riseup.net>]

I have a very WIP branch adding search paths to Lua.
https://git.sr.ht/~raingloom/guix-source/tree/raingloom/lua

The problem is that Lua uses a different path format compared to most
packages. Instead of a list of directories, it is a list of path
patterns, each potentially containing a question mark ('?'), which gets
substituted for the module being searched.

For pure Lua modules, this basically means /lib/lua/5.3/?.lua and
/lib/lua/5.3/?/init.lua.
I am not aware of any practical use of paths that do not contain a '?',
so there is a relatively easy solution here:
use the string.gsub function to expand every path component that does
not contain a '?' into two components, the ?.lua and the ?/init.lua one.
This means externally defined components are not expanded, only the
ones defined by Guix.

For C modules, it's a bit trickier, because there is a special
component that looks into loadall.so, which I must admit I'm not
familiar with. As far as I know, popular modules all define their own
shared object file, never a loadall.so.

Also, the path is separated by semicolons, not colons, but I think Guix
can already handle that.

The approaches I see:
- patch loadlib.c in Lua to expand LUA_PATH and LUA_CPATH according to
  the above mentioned heuristics, or
- patch Guix to support paths of this format
- define something like GUIX_LUA_PATH that gets expanded on its own and
  only contains Lua modules defined in Guix, and then gets appended to
  package.path (and do the same for package.path)


Solving this would open up access to the huge variety of Lua modules
and make things like game development in LÖVE much better integrated.
Its users could even use `guix pack` to distribute their games.