Update pubgrub to new add_incompatibility_from_dependencies

[konstin]

We had previously changed the signature of DependencyProvider::get_dependencies to return an iterator instead of a hashmap to avoid the conversion cost from our dependencies Vec to the pubgrub's hashmap. These changes are difficult to make in pubgrub since they complicate the public api. But we don't actually use DependencyProvider::get_dependencies, so we rolled those customizations back in pubgrub-rs/pubgrub#226 and instead opted to change only the internal add_incompatibility_from_dependencies method that we exposed in our fork. This aligns us closer with upstream, removes the design questions about DependencyProvider from our concerns and reduces our diff (not counting the github action) to +36 -12.