Refactor PathScanner to avoid Dir.glob

[casperisfine]

My main concern with Dir.glob is that it becomes sorted by default in 2.8/3.0 which makes it quite slower for large result sets. That can easily be fixed by passing sort: false, but while I was at it I figured there might be ways to avoid the cost of allocating and returning giant arrays.

So instead I use foreach which yield directory entries one by one.

I benched it against large directories and it's noticeably faster: https://gist.github.com/casperisfine/68819290350f8036d48a1d71777d5616

Calculating -------------------------------------
            original     18.595  (± 5.4%) i/s -    372.000  in  20.025736s
                 new     25.369  (± 3.9%) i/s -    508.000  in  20.037516s

Comparison:
                 new:       25.4 i/s
            original:       18.6 i/s - 1.36x  (± 0.00) slower