clanModules/importer: Init importer inventory module

Initialize the `importer` inventory module.
It is a power user module that allows for convenient, flexible,
structured imports for hosts.

It exposes the `extraModules` functionality of the inventory,
without any added configuration.

Closes: #2356

clanModules/flake-module.nix

@@ -13,6 +13,7 @@
     golem-provider = ./golem-provider;
     heisenbridge = ./heisenbridge;
     iwd = ./iwd;
+    importer = ./importer;
     localbackup = ./localbackup;
     localsend = ./localsend;
     matrix-synapse = ./matrix-synapse;