Add example failing test with canImport

[keith]

Currently canImport for modules that aren't in your deps returns
true, but then things fail because underlying clang modules aren't in
the dep tree.

[keith]

This seems like a lose lose to me we can either:

1. leave this behavior, but the error message is very bad. for example in this case of missing SwiftUI the error that you see ends up being:

<unknown>:0: error: cannot load underlying module for 'CoreData'
<unknown>:0: error: cannot load underlying module for 'CoreData'

2. we could make the canImport fail, but that could be worse if the user just forgot to add the dep