[2020-resolver] List downloaded distributions before exiting

[McSinyx]

This unifies the behavior of pip download (listing downloaded distributions before exiting) and fixes GH-8696.

[chrahunt]

I think it is probably reasonable to move successfully_downloaded to RequirementPreparer._prepare_linked_requirement. I would push back a little on getting rid of needs_more_preparation, because:

1. The scope of needs_more_processing is essentially RequirementPreparer, you don't need to look outside it to understand its impact
2. successfully_downloaded doesn't critically affect processing, just information display. Now that it is doing both it is harder to reason about.
3. it's not even clear to me that successfully_downloaded is needed (can we just print "Processed (all reqs)" in pip download?), and if we get rid of it then there's no need to merge it with anything :)

[McSinyx]

Thank you for the thoughtful comment, indeed successfully_downloaded only exists for historical reasons. I've adapted the patch accordingly.

[uranusjr]

I wonder if this is a good chance to change the output format as well. The current space-delimited format is very difficult to parse, both visually or by a program.

The same actually applies to the “successfully installed” message for pip install as well. Would a PR changing it to, say, one package on a line, be welcomed?

[pradyunsg]

Separately, yes.

[McSinyx]

@uranusjr, I've filed the ticket above to further discuss how we might want to improve it. BTW since you are here, may I have a review from you as well?

[uranusjr]

I am a little (very little) worried about dropping the if req_to_install.editable part, but can’t find a code path that makes a difference. I’ll err on the side of cleanup and agree with the logic change. We can always fix the condition if that turns out to not work.