This is to simplify the workflow for the time traveling aspect of `--publish-time`.
You can go into a repo and set a time and get the results as if it is
that time.
You don't need to look up the min-publish-age and do the math manually.
If there are multiple registries, that becomes even more difficult.
Granted, this is of limited use while it is a flag on `cargo
generate-lockfile`.
I could see moving this to `resolver.now` or something.

…lang#17328)
### What does this PR try to resolve?
One of the concerns is can we be transparent about how things are
resolving. We are already helped by
- error messages
- listing unused dependencies and how old they are
Now we'll report what the unused dependency is compared against.
Part of rust-lang#17009
### How to test and review this PR?
This is stacked on rust-lang#17327