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