swift: Fix duplicate SDK include paths causing a compile error

Some dependencies can bring include paths pointing to older macOS SDK's.
In this case, it was libffi pointing to SDK from 12.0. When the
Foundation
framework is imported in Swift, swiftc attempts to import the FFI module
from the most recent version of the SDK, which causes a compilation
error
because of conflicting definitions between the two SDK versions.

SwiftPM also had this problem:
https://github.com/swiftlang/swift-package-manager/pull/6772

The solution on our side is a simplified version of what SwiftPM did.
Let's naively look for .sdk paths in the compile args of our
dependencies
and replace them with the most recent one.

I included a test which is confirmed to fail without the workaround
added in this patch. This was not tested on anything else than macOS,
but I don't expect it to make the situation worse in any case.

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