More dependency graph resolver fixes

[jeffkl]

Bug

Fixes:

Description

These changes address issues found around transitive pinning and slowness when downloading packages.

When a package is transitively pinned, NuGet does not honor PrivateAssets=All and instead just adds those packages.

To make package downloads happen in parallel, the algorithm now queues a background thread which fetches the package. Later on, the item is taken off the top of the queue and the task is awaited. This makes all packages download in parallel for any given level like the legacy algorithm.

I also fixed an issue where the new algorithm was merging runtimes more than was necessary. It now uses a HashSet to keep track of which frameworks we have already processed runtime graphs for.

I've tested this with an experimental VS insertion and there is only one regression that we're going to get an exception for.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.