[Draft] Adding a Build Configuration Import Test

My guess is most often one will want to import exactly what one is testing (Brent example) but I guess it would be very handy to do something like

if canImport(Foo)
   import Foo.Bar as FooBar
#endif

I don't see any particular reason you couldn't support anything `import` can support in an `#if import`.

Also +1 on Haraviks point:

My only question is whether there should be a way to test minimum module versions? If I add support for a module but target a particularly recent version then that support won’t compile for platforms with older versions of the same module, so I should be able to prevent them from being a match.

I think we'd be better off extending our existing `@available` and `#availability` mechanisms to test module versions in addition to platform versions. Two small, orthogonal features are better than one big, confused one.

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Brent Royal-Gordon
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