packaging: consolidate CLI functionality into packaging.py Consolidating functionality for invoking code in the hgpackaging package through a single CLI entry point will make things simpler when we add more complexity to that package. For example, it will allow us to run things out of a virtualenv with third party packages. This commit consolidates functionality from the Inno and WiX build.py scripts into a new packaging.py script. That script simply creates a virtualenv and runs the CLI functionality in it. The new virtualenv is populated with jinja2 because I felt it easier to incorporate requirements file processing in this commit and we will soon use jinja2 in an upcoming commit. The unified CLI functionality will also make it easier to script other packaging workflows going forward. e.g. RPM, Debian, and macOS packaging. Differential Revision: https://phab.mercurial-scm.org/D7156