Improve duplicate imports

[johnynek]

I noticed if I made a duplicate import that does not exist to a name that collides with a predef name, the error message is bad.

I should add a test exercising this error message to be sure. There were two things:

1. we didn't see both imports, just one.
2. the predef import said "unknown source" which means that we need to handle predef separately in error messages.