Merge dev15.7.x to dev15.7.x-vs-deps

LOC CHECKIN | dotnet/roslyn dev15.7.x-vs-deps | 20180409

Merge dev15.7.x to dev15.7.x-vs-deps

Move to dotnet pack

Using the console outputs warning messages which MSBuild interprets as a
build warning and we fail the build due to warn as error being enabled.
Just hide the warnings until we can find a replacement setting.

https://github.com/NuGet/Home/issues/6817

Moved the last of our NuGet package creation over to dotnet. Deleted all
remaining infrastructure for using NuGet.exe.

Merge dev15.7.x to dev15.7.x-vs-deps

Moving off of nuget pack to dotnet pack instead. This opens the door for
us to do the packaging on non-Windows machines

Document the use of lazy initialization in TargetFrameworkUtil

Make HostAnalyzerManager a bit lazier

Enable building sourcebuild

Merge dev15.7.x to dev15.7.x-vs-deps

Avoid creating all metadata references in the TargetFrameworkUtil static constructor

Avoid IForegroundNotificationService interaction from tests that can't use it

Fixes #25930

Merge dev15.7.x to dev15.7.x-vs-deps

We take the analyzer references provided from VSIX extensions and create
a map of AnalyzerReference.Id -> AnalyzerReference, which is done by
cracking the metadata and getting the .NET assembly identity for them.
We were doing this on the UI during package load, which really isn't
necessary.