Fix symbol search in salsa

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

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