Handle glob imports in the migration generator

[m4tx]

Currently, having a glob import in a source file where a model lives will trigger a warning: "Glob imports are not supported". It would be nice to support glob imports somehow — for instance, by copying all of them into the migration file. (See #66 (comment))

This will probably be fixed by #67, but since there are easier ways to tackle this independently from said issue, I'm posting a separate issue here.