Add 'path' fetcher

This fetchers copies a plain directory (i.e. not a Git/Mercurial
repository) to the store (or does nothing if the path is already a
store path).

One use case is to pin the 'nixpkgs' flake used to build the current
NixOS system, and prevent it from being garbage-collected, via a
system registry entry like this:

  {
      "from": {
          "id": "nixpkgs",
          "type": "indirect"
      },
      "to": {
          "type": "path",
          "path": "/nix/store/rralhl3wj4rdwzjn16g7d93mibvlr521-source",
          "lastModified": 1585388205,
          "rev": "b0c285807d6a9f1b7562ec417c24fa1a30ecc31a"
      },
      "exact": true
  }

Note the fake "lastModified" and "rev" attributes that ensure that the
flake gives the same evaluation results as the corresponding
Git/GitHub inputs.

src/libfetchers/attrs.hh

@@ -34,4 +34,6 @@ std::optional<bool> maybeGetBoolAttr(const Attrs & attrs, const std::string & na
 
 bool getBoolAttr(const Attrs & attrs, const std::string & name);
 
+std::map<std::string, std::string> attrsToQuery(const Attrs & attrs);
+
 }