NuGet Restore support for PackageTargetFallback (f.k.a Imports)

[rrelyea]

Design posted: https://github.com/NuGet/Home/wiki/PackageTargetFallback-(new-design-for-Imports)

[rrelyea]

cc: @davidfowl

[brthor]

Is there a straw man for tool imports?

[emgarten]

@brthor can you share the id/versions of any tools that you know of that still require imports?

[brthor]

Perhaps @mlorbetske knows? You were mentioning a couple in the monday sync.

[mlorbetske]

Microsoft.composition 1.0.27 is the one we were discussing earlier