Diff between two package versions

I often check Packagist when I want to see which other package versions a specific package requires. A diff view between two packages listed on Packagist would make my work much easier when planning upgrades.

For example: when planning an upgrade for `api-platform/core` from 3.1 to 3.4, I switch back and forth in the long version list and try to remember package versions. A function like "Show me the difference of requirements between v3.1 and v3.4" would be great

No files, tests, or entry points are identified. Start by locating the Packagist package version and requirements views, then determine how two selected versions could be compared; done means a user can request a comparison such as api-platform/core v3.1 versus v3.4 and see the differences in required packages and versions.

Written by the indexing model from the issue text.