Fix package restore cycle detection

[drewnoakes]

Fixes #9050

In #8239 we added a feature that aimed to detect NuGet package restore loops, and to stop them after a certain number of iterations to prevent endless resource usage.

It turns out that feature never worked due to using a byte[] as a key in a dictionary.

This PR:

• Fixes the byte[] hashing issue by wrapping the byte[] with a new Hash type that adds Equals and GetHashCode, makes the type immutable and provides a little more type safety.
• Pulls all cycle detection code out into a single component, simplifying the PackageRestoreDataSource class somewhat.
• Bumps the compiler version we use so that we can start dogfooding C# 12 bits, and uses a primary constructor in two places.
• Changes the message displayed to the user to be hopefully clearer and specify exactly which project is causing the restore failure, which should make further investigation easier.
• Adds some unit tests for the restore detection. Writing these, I'm not sure the detection algorithm we use is the best fit. We will monitor telemetry and listen for feedback on false positives/negatives and iterate as needed.
• Updates namespaces across the code from an incomplete move of code from the VS layer to the host-agnostic layer in Move PackageRestoreDataSource to Microsoft.VisualStudio.ProjectSystem.Managed #8850, so that everything's consistent. Moves some unit test code in the same direction too.
• Removes some redundant using directives in the language service code that started appearing as warnings.

Before

After

Microsoft Reviewers: Open in CodeFlow