Adjust MultiTargets imports

[rolfbjarne]

From here:

@dsplaisted I changed it to only add to AfterMicrosoftNETSdkTargets if the file to be added actually exists: xamarin/xamarin-macios@bef4867, I suppose that should work as well?

It probably works, but it looks like the import path depends on a property that is set in the workloads, so when the workloads aren't imported the property is blank so its trying to import from a path relative to the workload manifest targets instead. So it's not checking the existence of the file it's actually trying to import, it's checking a different file path which will probably never exist.

It seems like it would be more precise to check if the property was set, or to move the logic to the import targets where you know it will be set.

Ref: dotnet/sdk#30230

[rolfbjarne]

We don't import the MultiTargets file anymore, so this doesn't apply.