Add user process to compare/sync/merge two or more packages

When adding new packages to DejaCode, there are often several URLs for the same package (for instance when adding PyPI packages there can be multiple wheels and sdist for the same package).

Then once the scans are completed, one may update one of the packages, usually to fill in fields that are blank.

Once this is done for one package it would be great to have a convenient way to compare, sync and possibly merge the new attribute values of the updated package with the other packages created for the same version, or perhaps even gather attribute values from previous versions of the same package.

Otherwise, the user typically needs to follow an error prone and work intensive sequence of actions using multiple browser tabs and copy/paste:

- in a new browser tab, find the most recent version of the same package and visually compare the field values with the tab that contains the recently updated package and selectively copy and paste some field values.
- in a new browser tab, open the other related packages and visually compare the field values with the tab that contains the recently updated package and selectively copy and paste some field values multiple times.

Some detail design is needed of course, but this would be a great enhancement to package maintenance usability. The recent enhancements to PurlDB for the `package_set` feature might be helpful here.