Handle the special case when publishing simple and structured and the upstream repo contains a distribution named "default"

[quba42]

Describe the bug
If an upstream repo contains a distribution named "default", and the user tries to publish this using structured=True AND simple=True, the structured "default" distribution will collide weith the simple "default" distribution and the publication will fail as follows:

    "error": {
        "description": "duplicate key value violates unique constraint \"core_publishedmetadata_publication_id_relative__60dd832b_uniq\"\nDETAIL:  Key (publication_id, relative_path)=(8b7f14bf-ca06-4c9b-9233-2e14edcf5322, dists/default/all/binary-amd64/Packages) already exists.\n",
        "traceback": "  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/pulpcore/tasking/pulpcore_worker.py\", line 380, in _perform_task\n    result = func(*args, **kwargs)\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/pulp_deb/app/tasks/publishing.py\", line 172, in publish\n    release_helper.finish()\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/pulp_deb/app/tasks/publishing.py\", line 293, in finish\n    component.finish()\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/pulp_deb/app/tasks/publishing.py\", line 216, in finish\n    package_index = PublishedMetadata.create_from_file(\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/pulpcore/app/models/publication.py\", line 281, in create_from_file\n    content.save()\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/pulpcore/app/models/base.py\", line 149, in save\n    return super().save(*args, **kwargs)\n  File \"/opt/rh/rh-python38/root/usr/lib64/python3.8/contextlib.py\", line 75, in inner\n    return func(*args, **kwds)\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django_lifecycle/mixins.py\", line 134, in save\n    save(*args, **kwargs)\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/models/base.py\", line 739, in save\n    self.save_base(using=using, force_insert=force_insert,\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/models/base.py\", line 776, in save_base\n    updated = self._save_table(\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/models/base.py\", line 881, in _save_table\n    results = self._do_insert(cls._base_manager, using, fields, returning_fields, raw)\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/models/base.py\", line 919, in _do_insert\n    return manager._insert(\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/models/manager.py\", line 85, in manager_method\n    return getattr(self.get_queryset(), name)(*args, **kwargs)\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/models/query.py\", line 1270, in _insert\n    return query.get_compiler(using=using).execute_sql(returning_fields)\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/models/sql/compiler.py\", line 1416, in execute_sql\n    cursor.execute(sql, params)\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/backends/utils.py\", line 66, in execute\n    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/backends/utils.py\", line 75, in _execute_with_wrappers\n    return executor(sql, params, many, context)\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/backends/utils.py\", line 84, in _execute\n    return self.cursor.execute(sql, params)\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/utils.py\", line 90, in __exit__\n    raise dj_exc_value.with_traceback(traceback) from exc_value\n  File \"/opt/theforeman/tfm-pulpcore/root/usr/lib/python3.8/site-packages/django/db/backends/utils.py\", line 84, in _execute\n    return self.cursor.execute(sql, params)\n"
    },

Expected behavior

Either handle this special case, or at least provide a meaningful error message!

Additional context

This is most likely to occur during Pulp to Pulp syncs, since I have never seen a repo in the wild with a distribution="default" anywhere outside of pulp_deb...