ResolveNuGetPackageAssets allocates 3.7% of solution-wide design-time build

[nkolev92]

From @davkean on September 26, 2017 12:55

This is literally turning text -> text, can we do this without NewtonSoft.Json? Otherwise, try the lower level reader API that JSON.NET has?

Copied from original issue: dotnet/sdk#1605

Moved from: NuGet/Home#6755

cc @davkean @nguerrera