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Here's a patch. Summary: any_missing() returns less bogus
modules.

- I've rewritten scan_code() more or less from scratch,
factored bits and pieces out for readability.
- keep track of global assignments and failed imports per
module; use this to determine whether the Y in "from X
import Y" is a submodule or just a global name. This is not
100% doable: you can't tell which symbols are imported when
doing a star import of a non-Python module short of actually
importing it.
- added a new method to ModuleFinder: any_missing_maybe(),
which returns *two* lists, one with certain misses, one with
possible misses. The possible misses are *very* often false
alarms, so it's useful to keep this list separate.
any_misses() now simply returns the union of
any_missing_maybe().

I pretty sure this is all 100% backward compatible, so after
some more testing it could be checked in.