rearrange conflict checking algorithm
    
    Instead of iterating each file in the package under test, and finding
    every package containing that file (which is a relatively slow operation
    in libsolv) -- we can instead iterate *every* other package and use
    Python set operations to look for any overlaps in filenames.
    
    This substantially improves the running time of check-conflicts when
    testing packages with a large number of files.
    
    Bug: 1465736
    Change-Id: I49634b18738d997f7b8596fcebd7abd09da93385