Don't process sources from nix store?

[roberth]

Store paths are typically already freshly fetched clean repos

Advantages:

• no interference from ~/.gitconfig etc
• smaller code path footprint for projects that use dependencies that use gitignore
  • much smaller impact for bugs like Unescape hash sign #22 (Nix 2.3 builtins.match behavior change)
• performance

Disadvantages:

• what if the repo gitignores some of its own contents...

Requirements:

• add way to add additional ignores #9 should still be possible

Unresolved:

• Where to implement this logic?
• Should this be implemented here or in pkgs.cleanSourceWith?