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.
It's a good idea to distinguish between absolute and relative paths at
the type level, to avoid accidental dependency on the cwd, which
really shouldn't matter for rust-analyzer service
