Detect cycle between root project and transitive dependency in new dependency resolver

[jeffkl]

Bug

Fixes: NuGet/Home#14052

Description

The new dependency resolver does not check if a transitive dependency happens to have the same name as the root project and instead replaces the root project in the graph with a dependency with a higher version. When the algorithm then tries to create a flattened graph, it throws a KeyNotFoundException trying to find the root project in the resolved graph.

This was unintentionally fixed in #6231 which made the root project always have its own ID but during resolution the algorithm now adds the package with the cycle and its detected during the flattening.

This change makes it so the name of the root project is indexed as 0 and then subsequent calls to index it will return the ID of the project. Then during resolution, there's a check to see a transitive dependency has a cyclic relationship with the root project and if so, leaves it out of the resolved graph. During the flattening of the graph, the cycle is then detected and an error message is emitted.

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.

[jebriede]

Approve with minor and optional suggestion on test comment.