Address feedback from original change to introduce new dependency resolution algorithm

[jeffkl]

Bug

Tracked at https://github.com/NuGet/Client.Engineering/issues/2959

Description

My original change to introduce a new dependency resolution algorithm was rushed and this change address the following feedback:

• See if we can get rid of the new exception handling path of AggregateException. This was only needed because some code paths call .GetAwaiter().GetResult() instead of await but we undid some of that during the final refactoring. (Introduce optimized dependency graph resolution algorithm #5963 (comment))
• Consider moving SharedStringBuilder to NuGet.Versioning instead of having it a "shared" compiled resource (Introduce optimized dependency graph resolution algorithm #5963 (comment))
• Use an inline suppression for ProjectRestoreCommand.GetRuntimeGraph() catching a general exception and remove it from GlobalSupressions.cs (Introduce optimized dependency graph resolution algorithm #5963 (comment))
• Fix whitespace in ProjectRestoreCommand.DownloadDependenciesAsync() (Introduce optimized dependency graph resolution algorithm #5963 (comment))
• Fix whitespace in RestoreCommand.ExecuteRestoreAsync() (Introduce optimized dependency graph resolution algorithm #5963 (comment))
• Consider using NoAllocEnumerator or a for loop in RestoreCommand.CreateFrameworkRuntimePairs() (Introduce optimized dependency graph resolution algorithm #5963 (comment))
• Reconsider marking properties in RestoreTargetGraph as internal and instead use a new constructor or init (Introduce optimized dependency graph resolution algorithm #5963 (comment))
• Use extension method for getting the TargetFramework from the PackageSpec (https://github.com/NuGet/NuGet.Client/pull/5965/files#r1715638641)
• Use new property from PackageSpec in PackageSpecReferenceDependencyProvider.cs (Implement transitive pinning in new dependency graph resolver #5965 (comment))

I broke each one out into its own commit if that's eaiser to review.

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]

Thanks for addressing all this feedback :)