hgext: turn 'hgext' into a namespace package
Actually since Python 2.3, there is some way to turn top level package into
"namespace package" so that multiple subpackage installed in different part of
the path can still be imported transparently. This feature was previously
thought (at least by myself) to be only provided by some setuptool black magic.
Turning hgext into such namespace package allows third extensions to install
themselves inside the "hgext" namespace package to avoid polluting the global
python module namespace. They will now be able to do so without making it a pain
to use a Mercurial "installed" in a different way/location than these
extensions.