Use overlay

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

overlay/default.nix

@@ -0,0 +1,14 @@
+final: prev:
+{
+  lib = prev.lib.recursiveUpdate prev.lib {
+    fetchers = {
+      sshKnownHosts = import ./fetchers/sshKnownHosts final prev;
+    };
+
+    runtime = {
+      merge = {
+        keyValue = import ./runtime/merge/keyValue final prev;
+      };
+    };
+  };
+}