lib/maintainers: rename file to maintainers-list.nix

Based on a request by @oxij:
“Can we also rename this file to `maintainers/maintainers-list.nix` while we at
this? Motivation: much saner `git log ./lib`.”

lib/default.nix

@@ -21,7 +21,7 @@ let
 
     # packaging
     customisation = callLibs ./customisation.nix;
-    maintainers = import ./maintainers.nix;
+    maintainers = import ./maintainers-list.nix;
     meta = callLibs ./meta.nix;
     sources = callLibs ./sources.nix;