Refine `source-dependencies/import` test.

Clarify what `source-dependencies/import` really tests by renaming
it to `source-dependencies/import-package` which better describes
the scope of that test.

Also, introduce analogous `source-dependencies/import-class` test which
tests dependency tracking on imported classes.

The reason why we should distinguish between imported packages and classes
is that incremental compiler doesn't track packages as products of source
files as it does for classes. Therefore, problem with imported classes
is much more tractable than with imported packages.