search: allow filtering by existence of store path

[cdmistman]

Why

allows filtering results by whether the store path exists

What changed

• added --filter-built flag
• optionally filter fuzzy search results by presence of the store path

Test plan

1. nix eval github:replit/rippkgs/cad/filter-available#lib.genRegistry --refresh --apply 'f: f builtins.currentSystem (import <nixpkgs> { config = { allowUnsupportedSystem = true; }; })' --impure --json >nixpkgs.json
2. nix run github:replit/rippkgs/cad/filter-available#rippkgs-index -- --output nixpkgs.sqlite --registry nixpkgs.json
3. nix run github:replit/rippkgs/cad/filter-available#rippkgs -- --filter-built --index nixpkgs.sqlite <something>
   • if you put <something> that you've previously built from the specific nix channel, then the result should appear
   • if you put <something> you haven't built, then the result should not appear. but if you remove the --filter-built flag, then the result should appear!