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.

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

···
+
+
+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

pkgs/development/interpreters/python/mk-python-derivation.nix

···
   postFixup = ''
     wrapPythonPrograms
   '' + lib.optionalString catchConflicts ''
-    # check if we have two packages with the same name in closure and fail
-    # this shouldn't happen, something went wrong with dependencies specs
-    ${python.interpreter} ${./catch_conflicts.py}
+    # Check if we have two packages with the same name in the closure and fail.
+    # If this happens, something went wrong with the dependencies specs.
+    # Intentionally kept in a subdirectory, see catch_conflicts/README.md.
+    ${python.interpreter} ${./catch_conflicts}/catch_conflicts.py
   '' + attrs.postFixup or '''';
 
   passthru = {
···
     isBuildPythonPackage = python.meta.platforms;
   };
 })
-
-