Always sort fieldpath string sets using custom comparator

Whenever we store sets of field paths strings (eg., ["a.b", "a", "c.d"]) use a custom string comparator that treats "." as the first character.

This is to protect us against future bugs like SERVER-66418.  It also makes the solution to that issue more efficient (because it avoids a set copy) and would allow O(n log n) solutions to many field path comparison functions that currently are O(n^2).  For example `isIndependentOf` and `isOnlyDependentOn` in https://github.com/mongodb/mongo/blob/master/src/mongo/db/matcher/expression_algo.cpp#L802