Fix build on non-macOS Apple platforms

[simonjbeaumont]

Motivation

When building this project for Apple platforms other than macOS, the build failed for a couple of reasons. First, NIOSSHServer requires Foundation.Process which is only available on macOS. Second, the tests are making use of an CryptoKit API that's only present on iOS 13.2 (and aligned other versions), which is newer than the 13.0 stated in the Package.swift:

error: Cannot find type 'Process' in scope
...
error: 'isValidAuthenticationCode(_:authenticating:using:)' is only available in iOS 13.2 or newer

Modifications

• Wrap all the code in NIOSSHServer with #if canImport(Foundation.Process) and, in main.swift, throw a fatalError with a helpful message.
• Add missing availability guards in test code. I've tried to keep them as scoped as possible.

Result

Source and tests can now build on Apple platforms other than macOS.

[dnadoba]

TIL that you can import check types and not only modules.

With that being said, why don't we use @available instead like we do in tests?

[simonjbeaumont]

Because that doesn't stop it trying to compile the code. If Foundation.Process was present on e.g. iOS but not available, then we could use @available here too, but it is simply not present in the iOS SDK (and others), so we must use a compiler directive.

[dnadoba]

Makes sense! Thanks