Modify ContentItemCollection.PopulateItemGroups to use pooling for highly allocated temporary data structures

[ToddGrun]

Bug

Fixes: NuGet/Home#13851

Description

Reduce allocations in ContentItemCollection.PopulateItemGroups by using pooling

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.

[ToddGrun]

Highlighted allocations the ones that are removed due to this change. Took two profiles of the before to verify it wasn't just a bad run:

[ToddGrun]

A couple thoughts:

1. If this codepath is going to soon be removed, let's just abandon this. Not executing code is the best optimization possible.
2. If the lock usage in SimplePool is a concern, it might be good to consider using something like Roslyn's ObjectPool.
3. If the possible increase in total memory footprint due to pooling is a concern (it hasn't been flagged as such in other VS codebases I've played in), then an option would be to clear out the pools once the work needing to use these objects is completed. Not a path I would like to go down, but thought I would call it out when listing potential concerns I could think of with this PR.

[microsoft-github-policy-service]

This PR has been automatically marked as stale because it has no activity for 7 days. It will be closed if no further activity occurs within another 90 days of this comment. If it is closed, you may reopen it anytime when you're ready again, as long as you don't delete the branch.