Make directories also match their descendants

[ilkecan]

From an end user perspective this makes inDirectory redundant.

Closes: #25

[ilkecan]

With this, it is not possible to only include a directory without its contents. Maybe inDirectory could be redefined to be able to use it to only exclude entries while keeping the directory. Like:

  exclude-descendants-but-keep-the-directory = rec {
    root = ./fixture1;
    actual = nix-filter {
      inherit root;
      include = [
        "src/innerdir"
      ];
      exclude = [
        (nix-filter.inDirectory "src/innerdir")
      ];
    };
    expected = [
      "src"
      "src/innerdir"
    ];
  };

[zimbatm]

thanks!