I18n: fixes to add-on message extraction

This commit fixes several issues in add-ons UI messages extraction code:

- In multi-file modules, the script would crash because it tried to write to
  the dir instead of a `translations.py` file;
- The add-on message extraction works by enabling the add-on, getting all
  messages; disabling the add-on, getting all messages; then comparing the
  two message sets. But often a bug happens where a class gets a
  description from somewhere else in memory. I couldn’t debug that, so a
  workaround is to check that the message isn’t a corrupted one before
  removing it;
- `printf()` doesn't exist in Python and would crash the script;
- `self.src[self.settings.PARSER_PY_ID]` can be replaced by `self.py_file`
  in class `I18n`, since a property exists to do that;
- At one point a generator was printed instead of its values, so let's
  unpack the generator to get the values. Maybe the print could be
  deleted entirely;
- Use SPDX license identifier instead of GPL license block, to be more in
  line with other scripts from the codebase.

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D15474

release/scripts/modules/bl_i18n_utils/bl_extract_messages.py

@@ -962,7 +962,12 @@ def dump_addon_messages(module_name, do_checks, settings):
     # and make the diff!
     for key in minus_msgs:
         if key != settings.PO_HEADER_KEY:
-            del msgs[key]
+            if key in msgs:
+                del msgs[key]
+            else:
+                # This should not happen, but some messages seem to have
+                # leaked on add-on unregister and register?
+                print(f"Key not found in msgs: {key}")
 
     if check_ctxt:
         _diff_check_ctxt(check_ctxt, minus_check_ctxt)