rely more on phedex to get file lists

DBS keeps track of files that may not actually exist at any site (unless they are globally invalidated by hand - rare), which leads to incorrect weight and hosting information. Instead, we can rely primarily on PhEDEx (which only knows files that actually exist) and just use DBS to get the number of events per file.

Tested with some Summer16 DY samples and output is identical with old and new scripts, as expected.