Provide cross platform API for downloading

[KathleenDollard]

NuGet.exe (Core)

dotnet install tool is using a temp project to download packages from NuGet because there isn't a .NET Core client that can do this.

We have found it difficult to hide this detail (hack) within the user experience (see https://github.com/dotnet/cli/issues/8707 for example).

It would be great to remove the temporary project in the near-ish future (sometime after 2.1.300) and explicitly download the required packages.

[nkolev92]

Duplicate of #5919.
It got de-prioritized at some point.

Compatibility checking would be a challenge here.
Not going to be straightforward to make the proj go away.

Now that I understand the background better, I'll look into #6630 to clean it up.