feat(nix/sparseTree): get a directory with only selected children r/2830

Given a path (which points to a directory and a list of paths which
are below that path, build a “sparse” version of that directory, so
that it only contains the listed paths (and their children):

    $ nix-build -E 'with import ./. {}; nix.sparseTree ./. [
        ./default.nix
        ./nix/readTree
        ./nix/buildLisp
        ./third_party/nixpkgs
        ./third_party/overlays
      ]'
    /nix/store/0ynj0gc613fs6mfp9snqcvdj5gfxbdzg-sparse-depot
    $ lr -t 'type == d' result/
    result/
    result/nix
    result/nix/buildLisp
    result/nix/buildLisp/example
    result/nix/readTree
    result/nix/readTree/tests
    […]
    result/third_party
    result/third_party/nixpkgs
    result/third_party/overlays
    result/third_party/overlays/haskell
    result/third_party/overlays/haskell/patches
    result/third_party/overlays/patches

This is useful if a derivation depends on depot.path (e. g. if it wants
to import depot at runtime). Usually this means that on every depot
commit (or even worse, every change of .git on a local machine), this
derivation has to be rebuild. By using sparseTree you can instead depend
on a stripped down version of depot which only contains the bits you
actually depend on, avoiding unrelated rebuilds.

Change-Id: I127b108c8b177c657fb46786d0a6256516fd2c52
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3503
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

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