Update nuget-buildtasks from upstream's `dev15.1` branch (#4703)

* Update nuget-buildtasks from upstream's `dev15.1` branch

dev15.1 branch + fixup import paths

This was prompted by needing to support nuget restore for projects with
`PackageReference`.

* [nuget] Bump reference to NuGet.BuildTasks to pick up a build fix

diff --git a/external/nuget-buildtasks b/external/nuget-buildtasks
index 04bdab55d8de9edcf628694cfd2001561e8f8e60..a348a006f5074d93b7331f8e0ceee3d1b20e798f 160000 (submodule)
--- a/external/nuget-buildtasks
+++ b/external/nuget-buildtasks
@@ -1 +1 @@
-Subproject commit 04bdab55d8de9edcf628694cfd2001561e8f8e60
+Subproject commit a348a006f5074d93b7331f8e0ceee3d1b20e798f