Perform download after resolving version

[colin-grant-work]

What it does

This PR adjusts the order in which operations are performed when downloading plugins. The resolution of version information is now followed by the download step for each plugin. Previously all dependency versions would be resolved before any downloads were executed. The parallel or no-parallel option is respected for all network calls.

How to test

1. Run yarn download:plugins and yarn download:plugins --no-parallel. Observe that logs are as expected.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I can confirm that the changes work as expected, both --parallel (default) and no-parallel work well.

I also confirm that:

• plugins are correctly downloaded
• packs are correctly resolved
• dependencies are correctly resolved
• plugins are correctly skipped if previously downloaded when attempting to resolve packs or dependencies
• the exclusion list is respected for all steps

[ccorn]

This certainly looks good. I'd like to try this with a slow connection, but at my current place, connectivity is fast. It might take two days before I get back, so I cannot carry out the tough test for now. Please do proceed.

[ccorn]

For quick testing, I have merged

• repo: update builtins #11113 at 8d31da9
  (waiting for approval at the time of this writing)
• this PR at 9a10b6b

and I can confirm that things work as they should, producing identical contents of the plugins folder for --parallel and --no-parallel.

Note that my output includes the message

- ms-vscode.references-view: already downloaded - skipping

which should be thanks to #11113. Otherwise, I suppose that there would have been two downloads of that extension under different plugins subfolder names, with different versions, resulting in 77 sub-folders instead of 76.

yarn download:plugins durations with a 50-Mbps connection:

Option	Duration
--no-parallel	241s
--parallel	45s

I will try it with a slow connection in a few days.
Thanks for the awesome work!