Clean and separate Glob and ERB processing

I was able to split the importer into 2 clear concerns, Globbing and ERB.

Sprockets API-wise I was able to remove most of the deprecated or private calls. Pretty close to being able to allow for Sprockets 3.x!

The nice big change is that the importer is completely hands off if the import is neither a glob or erb file. This avoids the overhead for the case of a simple import. Those calls just do a `super`.

I'd still love it if we could get globs in sass core somehow. That at least remove all the bad dependency tracking hacks here.

/cc @rafaelfranca @matthewd 
A bit of an inefficiency here. We always have to do a `find_filename` to get the full path. If we don't use it, `super` will end up re-finding the same thing. Repeated work.