Be more specific when reporting import errors

Currently, with code such as:

import Blahblah1 -- does not export foo
import Blahblah2 (foo)

bar = Blahblah1.foo

We simply get an error saying that Blahblah1 does not export foo. But we could do suggest more fine-grained information:

• suggest that the user might have mis-spelled Blahblah2 (as that module does export foo),
• perhaps Blahblah1 defines foo but does not export it, so we could suggest that foo be added to the export list of Blahblah1 (perhaps only if Blahblah1 is from the current package).