fix: add error message for github PR url in dep

[joshka]

Prior to this, using a github PR URL would cause cargo to attempt to
fetch from an incorrect URL several times before failing.
Providing a github pull request url now fails with an error message
that shows how to fix the problem.

E.g.:

bar = { git = "https://github.com/foo/bar/pull/123" }

Now gives the following error message:

dependency (bar) specifies a GitHub pull request link. If you were trying to specify a specific github PR, replace the URL with the git URL (e.g. `git = "https://github.com/foo/bar.git"`) and add `rev = "refs/pull/123/head"` in the dependency declaration.

Fixes: #15001

[rustbot]

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[weihanglo]

Thanks! If needed, we can iterate it later :)

[joshka]

I'm not quite sure what the right approach to getting the tests to succeed both in CI and locally for this change. Any guidance?

[epage]

Sorry I've not been able to get back to this and thanks @weihanglo for picking it up.

A non-blocking thought (since some improvement is better than no improvement) is to offer something similar for cargo add --git URL