Collect all path inclusion logic into the Path Classifier Class

[jcampbell05]

One thing I've noticed is over time we've ended up with logic for detecting which paths should be included in two spots.

For example here:
https://github.com/igor-makarov/xcake/blob/master/lib/xcake/generator/target_file_reference_generator.rb#L23

and here:
https://github.com/igor-makarov/xcake/blob/master/lib/xcake/path_classifier.rb

We should probably make sure or code to do we determining if a file should be included should belong in one class so then it's easy to determine when Xcake includes a file at one glance.