Use overlay

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

overlay/default.nix

@@ -0,0 +1,17 @@
+final: prev:
+let
+  callPackage = prev.lib.callPackageWith { pkgs = prev; };
+in
+{
+  lib = {
+    fetchers = prev.fetchers // {
+      sshKnownHosts = callPackage ./fetchers/sshKnownHosts { };
+    };
+
+    runtime = {
+      merge = {
+        keyValue = callPackage ./runtime/merge/keyValue { };
+      };
+    };
+  };
+}