Add a public target to NuGet.targets that can be referenced using BeforeTargets

[emgarten]

The public Restore target is called on the solution metaproj when restoring a solution. Within each project the top level target called is _GenerateRestoreProjectSpec

To allow users to perform tasks before and after restore NuGet.targets should add a public target that can be used with BeforeTargets and AfterTargets

The fix for this could be as simple as renaming _GenerateRestoreProjectSpec and adding a comment in NuGet.targets if we decide to stay with that target.

https://github.com/dotnet/cli/issues/5683

[dsplaisted]

Related to this, generating an error in a target that runs before _GenerateRestoreProjectSpec doesn't actually cause the restore operation to fail. I've filed #5309 for this.

[emgarten]

This has been added with CollectPackageReferences

[ashmind]

This has been added with CollectPackageReferences

Does this mean we need to attach to CollectPackageReferences instead of Restore, or just that it was implemented together with CollectPackageReferences?

[nkolev92]

@ashmind
Attach to CollectPackageReferences.
In the solution case, restore is run on a metaproj, so the restore target can't be used as an extension point for projects.

Additionally, it's the only target that's consistent among VS and commandline for restore.