Build an async IVsPackageSourceProvider service

[kartheekp-ms]

Child task for #8675

[nkolev92]

Discussed in #9811 and https://github.com/dotnet/roslyn/pull/46022/files#r456003371 since it causes UI delays.

[zivkan]

FWIW, I don't support making an async version of IVsPackageSourceProvider as a direct port. From an API design point of view (think of the developer experience using the API), it's weird that we have a service/provider that does nothing more than get sources. It harms API discoverability, as the developer needs to hunt to find what other interfaces NuGet exposes and whether those are useful to their project.

With GetInstalledPackages, I created a INuGetProjectService which should be suitable for all project-related APIs. I propose we consider creating a INuGetSolutionService for all solution APIs.

As I mentioned in the duplicate issue, PackageReference projects can have additional per-project sources. Therefore we should consider whether a replacement API is per-project, solution-wide, or if we create two APIs.

[nkolev92]

pri3 according to @aortiz-msft

[nkolev92]

I think there's still a need for this service, but the primary problem in #8675 is that the NuGet is being loaded on the UI thread, and that's something that's been fixed.

[nkolev92]

https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1556209 related.