Refactor/driver

[leissa]

This is an intermediate step for #185

Changes

• Loading plugins as well as managing Passes, Normalizers, Backends, and search paths is being dealt with by Driver.
  This requires way less bookkeeping and boilerplate to load plugins.
• New option -l to list search paths in order
• Made a couple of members private and added proper getters + setters
• We now have namespace fs = std::filesystem
• Some minor things here and there like making things more consistent or removing unused stuff
• I started using the term "plugin" instead of "dialect". This is not consistent yet and will come in a future PR as I didn't want the diff going bonkers.