Common platform selected for native source sets

[IgnatBeresnev]

Not sure if it should be classified as a bug, but this behaviour first appeared in 1.6.20, presumably due to #2431 (just an assumption)

Accidentally noticed it when generating documentation for skiko: with Dokka >= 1.6.20 there are significantly fewer purple colored native targets, i.e macos used to be a native target (i.e purple) and now it's common

I've narrowed it down to sourceSets like macOsMain now returning multiple targets:

1.6.10

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1.7.20-dev-181 (same for 1.6.20)

[IgnatBeresnev]

Closed in favour of #2779, which is supposed to redesign the selector as a whole