Print shorter login URL in the terminal

It's sometimes hard to copy the URL from the terminal when authorizing `pub publish`. The URL has a lot of parameters and is very long, thus, some terminal emulators might break the lines and make it hard to copy.

This causes issues such as: https://github.com/dart-lang/pub-dev/issues/2611

As @isoos suggested in https://github.com/dart-lang/pub-dev/issues/2611#issuecomment-518245602 we could mitigate by printing `http://localhost:/login` in the terminal, and when the user opens this URL we can redirect to the correct sign-in URL, which then in turn redirects back to `localhost`.

This makes the URL to be copied much shorter, and doesn't introduce any state on `pub.dev`.
If the user can't open `localhost` in the browser, the login flow won't work anyways, as this is the callback URL.

Start by tracing the pub publish authorization flow and where its login URL is printed in the terminal. Done means the terminal shows a short localhost login URL and opening it completes the existing browser redirect flow without requiring state on pub.dev.

Written by the indexing model from the issue text.