python: Move catch_conflicts.py into subdirectory (#23600)

Python does add the script's directory into "sys.path". For the case of
"catch_conflicts.py" this means "/nix/store" is added to "sys.path". This can
result in very long delays if the store contains a lot of entries.

(moved from master commit 76213d102c)

pkgs/development/interpreters/python/catch_conflicts/README.md

@@ -0,0 +1,13 @@
+
+
+catch_conflicts.py
+==================
+
+The file catch_conflicts.py is in a subdirectory because, if it isn't, the
+/nix/store/ directory is added to sys.path causing a delay when building.
+
+Pointers:
+
+- https://docs.python.org/3/library/sys.html#sys.path
+
+- https://github.com/NixOS/nixpkgs/pull/23600