chore: restructure source sets to be hierarchically correct

[ianbotsf]

Issue #, if available:

(none)

Description of changes:

This change restructures our specialized Native source sets to properly depend on the main Native source set. In other words, theposix, desktop, and windows source sets now depend on the native source set. This is useful for being able to locate actual declarations which may be in non-leaf source sets.

For example, if the native source set has an expect declaration, the corresponding actual can now be found in posix in addition to in linuxX64, macosArm64, etc.

This change also updates the posix source set to include all Apple targets (e.g., iOS), not just MacOS targets.

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