Figure out relationship between Directory.Build.targets and workloads

[dsplaisted]

Currently, it's not possible to set a property in Directory.Build.targets which enables a .NET SDK workload. We should figure out what to do here - something will have to change (and we'll have to accept some sort of breaking change) in order to make Windows Forms / WPF support a .NET SDK workload, which we want to do eventually.

This is a tracking issue for now, we plan to put up a design on dotnet/designs for discussion. Related PRs:

#14393
#15449

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[marcpopMSFT]

Note that if we do add additional automatically imported targets, we'll want to double check performance impact. A quick check indicated that disabling the directory.build.props and directory.build.targets imports could save on the order of milliseconds so probably fine.

[marcpopMSFT]

No longer needed as workloads worked around this for now.