When NetPrevious is set to empty, some projects have unexpected behavior

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

When `NetPrevious` and `NetMinimum` are the same, the current behavior is to null out `NetPrevious`. This avoids duplicate-TFM conflicts for projects that target both `NetPrevious` and `NetMinimum`. The bad news is that this will cause errors for single-targeted projects that just target `NetPrevious`. It's worse new for projects that multi-target target `NetPrevious` and `NetCurrent`/some other non `NetMinimum` TFM.

A solution here is that:
- `NetPrevious` should always be set
- The only set of properties you can always combine are `NetPrevious` and `NetCurrent`
- A property should be introduced that is the union of `NetPrevious` and `NetMinimum`.