packaging: extract invocation of pyoxidizer to own function

I'll be refactoring how the WiX installer creation calls into
pyoxidizer and will need a lower level function for facilitating
that.

The new run_pyoxidizer() builds our execution environment
(with gettext available) and invokes pyoxidizer.

Differential Revision: https://phab.mercurial-scm.org/D10687