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

pkgs/development/interpreters/python/catch_conflicts/catch_conflicts.py

@@ -0,0 +1,30 @@
+import pkg_resources
+import collections
+import sys
+
+do_abort = False
+packages = collections.defaultdict(list)
+
+for f in sys.path:
+    for req in pkg_resources.find_distributions(f):
+        if req not in packages[req.project_name]:
+            # some exceptions inside buildPythonPackage
+            if req.project_name in ['setuptools', 'pip', 'wheel']:
+                continue
+            packages[req.project_name].append(req)
+
+
+for name, duplicates in packages.items():
+    if len(duplicates) > 1:
+        do_abort = True
+        print("Found duplicated packages in closure for dependency '{}': ".format(name))
+        for dup in duplicates:
+            print("  " + repr(dup))
+
+if do_abort:
+    print("")
+    print(
+        'Package duplicates found in closure, see above. Usually this '
+        'happens if two packages depend on different version '
+        'of the same dependency.')
+    sys.exit(1)