[merge] use lazy imports

[Andrew Bennetts]

Martin Pool wrote:
[...]
> 
> One downside with this is that tools like pyflakes don't understand it,
> so will not be able to give useful errors on missing imports.  On the
> whole that's something I'm prepared to give up: they don't catch many
> things that aren't caught by our tests, and they do generate a lot of
> spurious warnings.  

It should be possible to teach pyflakes to treat the lazy imports like real
imports, with a bit of effort.  Failing all else, you could write a script that
just copies a .py file minus the lazy_import magic, and run pyflakes on that.  

So I agree it's not a big deal that pyflakes as-is doesn't understand it.

-Andrew.