Support optional Nasal modules in fgdata/Nasal/MyModuleFoo

Support optional Nasal modules in fgdata/Nasal/MyModuleFoo
- Allow structuring generic Nasal sources into modules (i.e. support
  separate source folders for multiplayer/local weather/... sources)
- Option to enable/disable loading of generic Nasal modules (i.e we only
  need to load stuff that is really needed/enabled).
- Provide basic loading sequence: subfolders are strictly loaded _after_
  the base directory. So submodules can rely on .nas-scripts like math.nas,
  io.nas, debug.nas, ... to be already loaded - dropping the need for
  awkward listeners/timers to delay module initialization (loading order
  within Nasal/*.nas is still random)