lazy import aware branch of pyflakes

[Michael Hudson]

I got tired of cleaning up the imports of a couple of bzrlib files by
hand or by converting lazy imports to real imports, running pyflakes
and then lazifying the imports again, so I came up with a better way:
hacking pyflakes to understand lazy imports.

The branch at:

    https://code.edge.launchpad.net/~mwhudson/pyflakes/lazy-import-support

does three things:

 1) Parses lazy imports so that uses of the imported names don't give
    'undefined name' warnings.
 2) Warns if a lazily imported name is used at module level.
 3) Warns for names that are imported eagerly but *not* used at module
    level and so could be lazy.

It produces rather a for bzrlib currently (though some of it is noise:
pyflakes complains at this sort of thing:

import bzrlib.a_module
import bzrlib.another_module

which is probably a bug that should be fixed and submitted upstream).
Unless someone beats me to it, I'll probably submit branches that
clean some of this up, but the real value of a tool like this is
running it regularly to stop unused import cruft gathering, and I'll
try to do that too.

Cheers,
mwh