[PULP-1118] Add better error handling for repover duplicate content

[pedro-psb]

Added a proper error class for duplicate content handling and some more logging to inform exactly what are the conflicting content.

When duplicates are detected, we do some extra work to collect duplicate content.
A simple performance test shows it's not too bad:

In [1]: import pulpcore.plugin.repo_version_utils as ut

In [2]: content_qs = Package.objects.all()

In [3]: unique_keys = Package.repo_key_fields

In [4]: content_qs.count()
Out[4]: 24547

In [5]: ut.count_duplicates(content_qs, unique_keys)
Out[5]: 7701

In [6]: %timeit ut.count_duplicates(content_qs, unique_keys)
35.1 ms ± 154 μs per loop (mean ± std. dev. of 7 runs, 10 loops each)

In [7]: %timeit ut.collect_duplicates(content_qs, unique_keys)
61.4 ms ± 266 μs per loop (mean ± std. dev. of 7 runs, 10 loops each)

Closes: #7184

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough

[dralley]

It would seem like this assumes that a RepositoryVersion creation failed due to duplicates always? Do we want to assume that? Should the error be more specific?

[pedro-psb]

Yeah, makes sense. I'll make more specific one