awesome: refactor LUA_PATH env var and don't expose it at runtime

This commit partly reinstates changes from 5465d6f that had been somehow
reverted in 17d3eb2.  Also, a comment has been added in the hope that future
changes won't do the same.

Additionally, refactor the LUA_PATH env var to ensure that the internal lgi lua
files can't be required explicitly and to avoid possible name clashes (this
fixes issue #60232).

Finally, rather than using prepending `?.lua` append `;;` to LUA_PATH. Quoting
@psychon:

> This is interpreted by Lua as "add the default search path here" (which does
> indeed contain ?.lua, but also contains more).

Testing done:

- Build with `nix-build -I /path/to/repo -A awesome`
- Start an X session with xterm only
- Start xephyr, e.g. `Xephyr :1 -name xephyr -screen 512x384 -ac -br -noreset &`
- Run awesome like `DISPLAY=:1.0 ./result/bin/awesome`. Additionally, add
  `--search` options to expose lua modules that have a name clash with lgi's
  internal ones (see #60232 for more details) and `require` them in `rc.lua` to
  prove that they are loaded correctly

pkgs/applications/window-managers/awesome/default.nix

@@ -42,17 +42,17 @@ with luaPackages; stdenv.mkDerivation rec {
   cmakeFlags = "-DOVERRIDE_VERSION=${version}";
 
   GI_TYPELIB_PATH = "${pango.out}/lib/girepository-1.0";
+  # LUA_CPATH and LUA_PATH are used only for *building*, see the --search flags
+  # below for how awesome finds the libraries it needs at runtime.
   LUA_CPATH = "${lgi}/lib/lua/${lua.luaversion}/?.so";
-  LUA_PATH  = "?.lua;${lgi}/share/lua/${lua.luaversion}/?.lua;${lgi}/share/lua/${lua.luaversion}/lgi/?.lua";
+  LUA_PATH  = "${lgi}/share/lua/${lua.luaversion}/?.lua;;";
 
   postInstall = ''
     wrapProgram $out/bin/awesome \
       --set GDK_PIXBUF_MODULE_FILE "$GDK_PIXBUF_MODULE_FILE" \
       --add-flags '--search ${lgi}/lib/lua/${lua.luaversion}' \
       --add-flags '--search ${lgi}/share/lua/${lua.luaversion}' \
-      --prefix GI_TYPELIB_PATH : "$GI_TYPELIB_PATH" \
-      --prefix LUA_PATH ';'  "${lgi}/share/lua/${lua.luaversion}/?.lua;${lgi}/share/lua/${lua.luaversion}/lgi/?.lua" \
-      --prefix LUA_CPATH ';' "${lgi}/lib/lua/${lua.luaversion}/?.so"
+      --prefix GI_TYPELIB_PATH : "$GI_TYPELIB_PATH"
 
     wrapProgram $out/bin/awesome-client \
       --prefix PATH : "${which}/bin"