lib/pkg-lists: move to common location

Extract the helper functions defined in `efmls-configs-pkgs` to a common
location where they can also be used by none-ls's package list.

lib/pkg-lists.nix

@@ -0,0 +1,23 @@
+# This file isn't (currently) part of `lib.nixvim`, but is used directly by `efmls` and `none-ls` pkg lists
+lib: rec {
+  # Produces an attrset of { ${name} = name; }
+  topLevel = names: lib.genAttrs names lib.id;
+
+  # Produces an attrset of { ${name} = null; }
+  nullAttrs = names: lib.genAttrs names (_: null);
+
+  # Produces an attrset of { ${name} = [ scope name ]; }
+  # Where the "scope" is the (nested) attr names,
+  # and "name" is the value.
+  # If the name value is a list, it will be expanded into multiple attrs.
+  scoped = lib.concatMapAttrs (
+    scope: v:
+    if builtins.isAttrs v then
+      lib.mapAttrs (_: loc: [ scope ] ++ loc) (scoped v)
+    else
+      lib.genAttrs (lib.toList v) (name: [
+        scope
+        name
+      ])
+  );
+}