demandimport: update obsolete comment

demandimport: add support for PyPy PyPy's implementation of __import__ differs subtly from that of CPython. If invoked without a name or fromlist, it throws an ImportError, whereas CPython returns a reference to the level-appropriate importing package. Here, we achieve the same behaviour by hand.

test-demandimport: ensure that relative imports are deferred This adds a test not just at our local "top level" (the mercurial package), but also one level deeper (mercurial.hgweb).