Workaround NuGet breaking change

[ericstj]

NuGet removed Manifest.Save(Stream, bool) in the latest version.
Since NuGet is part of MSBuild our task is forcibly upgraded to the
latest version when running in the latest SDK. This results in a
missing method exception.

Workaround it by calling an API that NuGet didn't remove. This can
potentially cause failures in GenerateNuSpec because it will now run
Validation, however anyone using the Pack task in our package should
already be passing validation since NuGet runs the same validation
when reading the Manifest.

Replaces #2004.

[ericstj]

I've tested on CoreFx and it fixes building packages with the latest SDK. Our original use of the broken API comes from our use of NuProj, which still has this bug. As I mentioned above, we were still running validation during Pack for all of our scenarios so the risk of a break from this change is pretty low.

[tmat]

@ericstj Why can't we use the existing PackTask to generate NuSpec? What is it missing?

[tmat]

(Sorry, closed the PR by accident)

[ericstj]

@tmat see #1033. NuGet actually doesn't have a good replacement for GenerateNuSpec, but its possible that Pack might support enough inputs that we could avoid the NuSpec intermediate. We'd have to see when making those fixes.

[ericstj]

Just for reference (and for folks doing a callstack search) this is fixing:

System.MissingMethodException: Method not found: 'Void NuGet.Packaging.Manifest.Save(System.IO.Stream, Boolean)'.
   at Microsoft.DotNet.Build.Tasks.Packaging.GenerateNuSpec.WriteNuSpecFile()
   at Microsoft.DotNet.Build.Tasks.Packaging.GenerateNuSpec.Execute()

That was caused by NuGet/NuGet.Client#2438.

We're working around it by calling an API that was not removed.

[ViktorHofer]

Thanks for fixing!