NuGet package diagnostic warnings are now shown in the Solution window.
The diagnostic warnings are shown underneath the NuGet package with
a warning icon. Hovering over the warning icon will show a tooltip.
If the NuGet package is at the top level then a warning icon is
shown for the NuGet package and the tooltip message will show the
diagnostic message if only one diagnostic is associated with the
NuGet package.

Note that diagnostic messages can be associated with a NuGet package
at any level in the hierarchy. Whilst these are displayed in the
Solution window it is difficult to find them since there is no
indicating there is a warning higher up the tree.

Fixes VSTS #549720 - Display NuGet package dependency diagnostics in
Solution window