lua*Packages: Consolidate separate setup hooks together

- Lua packages now consistently use LUA_PATH/LUA_CPATH rather than a mix
  of those and NIX_LUA_PATH/NIX_LUA_CPATH
- Lua libraries are now consistently only added to the search path
variables if:
    1) The library actually has a corresponding directory to search
    2) The library is not already present in the search path
  This should help prevent the search paths from growing overly large
- Fixed bugs in some path helpers
- Changed the affected shell script indentation to 2 spaces; nixpkgs
  shell scripts are inconsistently split between 2 and 4 space
  indentation, but 2 matches better with the Nix expressions, so IMO it
  makes more sense