Use #if canImport(Darwin) where possible

[FranzBusch]

No description provided.

[glbrntt]

Do we want to keep the elseif here?

I think we should standardise on a pattern where possible. In other places we have:

#if canImport(Darwin)
import Darwin
#else
import Glibc
#endif

In other places I've seen #elseif canImport(Glibc)

[FranzBusch]

This one is all over the place and IMO something to tackle separately.

[o-nnerb]

There are still some places where you can change to canImport(Darwin).

I was updating my fork for a contribution, but I saw your PR. There are same statements about os restriction that are in different orders.

1. os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
2. os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
3. os(macOS) || os(iOS) || os(watchOS) || os(tvOS)

And there are statements like this !os(iOS) && !os(tvOS) && !os(watchOS) where you can updates to !canImport(Darwin) || os(macOS).

Nice job! I didn't think refactoring in this way would give the same results as adding || os(xrOS) 😅.