packaging: establish hgpackaging package Previously, contrib/packaging behaved as a root to a package directory and we had a "packagingutil" module. As I work more on packaging code, we'll want to have more code shared between different packaging tools. I think it makes sense to have a single package containing multiple modules than multiple top-level modules. This commit establishes an "hgpackaging" package by moving the existing packagingutil code to it. Differential Revision: https://phab.mercurial-scm.org/D6083