Issue Description:

In the compiler, we have several different diagnostics for module import errors. One common one that people hit is: "no such module 'X'" (sema_no_import). This is not very helpful. However, we do have a newer diagnostic that is more informative: "could not find module 'X' for target 'armv7-apple-ios'; found: arm64, arm64-apple-ios" (sema_no_import_target).

For places in the compiler which emit sema_no_import diagnostics on target mismatch, we should plumb down target information and display it using sema_no_import_target.