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