Previous solution for binning paths into disjoint directories was
simple and fast -- just a single binary search.
Unfortunatelly, it wasn't coorrect: if the ditr are
/d
/d/a
/d/c
then partitioning the file /d/b/lib.rs won't pick /d as a correct
directory.
The correct solution here is a trie, but it requires exposing path
components.
So, we use a poor man's substitution -- a *vector* of sorted paths,
such that each bucket is prefix-free
closes#5246
The task of `partition` function is to bin the flat list of paths into
disjoint filesets. Ideally, it should be incremental -- each new file
should be added to a specific fileset.
However, preliminary measurnments show that it is actually fast enough
if we just optimize this to use a binary search instead of a linear
scan.
