Add locateDominatingFile lib function

lib/filesystem.nix

@@ -0,0 +1,26 @@
+{ # locateDominatingFile :  RegExp
+  #                      -> Path
+  #                      -> Nullable { path : Path;
+  #                                    matches : [ MatchResults ];
+  #                                  }
+  # Find the first directory containing a file matching 'pattern'
+  # upward from a given 'file'.
+  # Returns 'null' if no directories contain a file matching 'pattern'.
+  locateDominatingFile = pattern: file:
+    let go = path:
+          let files = builtins.attrNames (builtins.readDir path);
+              matches = builtins.filter (match: match != null)
+                          (map (builtins.match pattern) files);
+          in
+            if builtins.length matches != 0
+              then { inherit path matches; }
+              else if path == /.
+                then null
+                else go (dirOf path);
+        parent = dirOf file;
+        isDir =
+          let base = baseNameOf file;
+              type = (builtins.readDir parent).${base} or null;
+          in file == /. || type == "directory";
+    in go (if isDir then file else parent);
+}