Switch tool install to using PackageDownloader

[sfoslund]

At the moment, .NET tool install uses its own package installer to download the packages it needs using a temp project and classic restore. Now that we have the NugetPackageDownloader we can make this code a lot cleaner (and potentially faster?) by switching the old tool install download logic to use the NugetPackageDownloader.

[nkolev92]

You'd need an assets file with the correct assets selected.
The downloader can't do that today.

The assets selection logic and assets file generation is tied into restore.

[sfoslund]

You'd need an assets file with the correct assets selected.

Ah okay. Do you know why tool install needs the assets file? I'll close this since it sounds like we won't be able to make this change.

[nkolev92]

Do you know why tool install needs the assets file?

That should be on SDK side. It should be something that's used when a tool is run.