migrate constructSystem to lib/

migrates the bulk of constructSystem to lib/ and splits out the largest
chunks into individual functions (namely SOPS, home-manager, and user creation);

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

lib/default.nix

@@ -2,6 +2,8 @@
 {
   # create rad-dev namespace for lib
   rad-dev = rec {
+    systems = import ./systems.nix { inherit lib; };
+
     # any(), but checks if any value in the list is true
     #
     # args: