Refactor the lua dependency checking.

This is in preparation for the next step, which is to find a suitable
lua interpreter, rather than just erroring when an interpreter is found
but doesn't have the necessary dependencies.

Helped-by: John Szakmeister <john@szakmeister.net>
Check dependencies for each lua interpreter.

- When USE_BUNDLED=OFF and the system has multiple lua interpreters
  (luajit AND lua) it may occur that only the later has the needed
  dependencies (lua-lpeg, lua-cmsgpack). If we pick luajit then
  finding the dependencies FAILS.
- This commit groups detection of the lua interpreter with the lua
  dependencies it tries to find them for both the interpreters and
  choses the first one that has them.

Helped-by: John Szakmeister <john@szakmeister.net>