Cherry-picked c973e15 to fix #60232 in the 19.03 branch. Hope this is the right way to do it. Thanks.

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 NixOS 19.03: Awesome Window Manager exposes lgi incorrectly in its LUA_PATH (regression) #60232 for more details) and require them in rc.lua to
  prove that they are loaded correctly

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.