Refactor new dependency graph resolution algorithm

[jeffkl]

Bug

Related: https://github.com/NuGet/Client.Engineering/issues/3008

Description

This is a major refactoring of the new dependency resolution algorithm to clean up the code and add more comments. Some of the changes include:

• Abstracting out major parts of the logic into separate methods so that the main ResolveAsync() method is easier to read
• Simplifying classes by trimming them down and combining them where it made sense
• Add XML doc comments to all members
• Make DependencyGraphResolver a partial class and move its child classes to separate files
• Went line-by-line and renamed any variable so that it aligned with the final implementation

To validate these changes, I've run restores against some key repositories and verified that the performance is the same and soare the outputs. All tests are passing and the only test updates I made were to improve reliability. One of the tests depended on a specific order of packages which is no longer deterministic due to parallelization.

Once this is merged, I will be working on a markdown file explaining the design and implementation of the algorithm for future contributors.

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.

[nkolev92]

This is such great work.

I don't really have any big feedback, but a bunch of ideas and comments that I might not even have if we weren't refactoring.

[donnie-msft]

Impressively well-documented!

[nkolev92]

🚢 🚢 🚢 🚢 🚢

[Nigusu-Allehu]

Thank you for the great documentation!