[SDK] Leverage NuGet TargetFramework Alias feature to improve multi-tfm experience

PR #3333 introduces multi-target support by having a generated project per TFM. Microsoft.NET.Sdk/10.0.300 will be introducing a new "TargetFramework Alias" feature was added which allows for user-defined TFM aliases.

https://github.com/NuGet/Home/blob/dev/accepted/2025/Multiple-Equivalent-Framework-Support-TFM-As-Aliases.md

This feature is important for us to leverage/support for two reasons:

1. PR #3333 assumes all resolved TFMs are unique. With TFM aliases they may no longer be unique.
2. We can leverage this feature ourselves to have a single generated project with an alias per outer-TFM alias.

Start by reading PR #3333 and the NuGet TargetFramework Alias proposal linked in the issue, then trace how generated projects represent and resolve each TFM. Done means aliases no longer cause duplicate resolved TFMs and the generated-project approach can represent a single project with an alias per outer-TFM alias.

Written by the indexing model from the issue text.