WiX v5 should(?) reject v4 extension Nupkegs

[barnson]

User story

As a setup developer, I can't accidentally use a WiX v4 extension with WiX v5, to avoid extensibility interface incompatibilities.

Proposal

WiX v5 should reject WiX v4 extension NuGet packages (those with only a wixext4 directory).

Considerations

Naturally, this is related to #7709 and I didn't forget it and create this one anew.

WiX v4 won't outright reject WiX v5 extensions but they should show up as damaged because of the different subfolder (wixext4 -v- wixext5).

However, it should be possible to create a single .wixext with both wixext4 and wixext5 directories with payloads to support both versions of WiX.