Implement a "project" MSI publication backend

The Windows MSI backend currently produces a single MSI for each application published.

In some situations, it may be desirable to publish a *single* MSI for a project that contains multiple apps.

There should be an alternate platform backend for Windows that allows for the production of a single project MSI.

Start by locating the existing Windows MSI backend and understanding how it publishes one MSI per application. Define the alternate platform backend's project-level entry point and determine how a project containing multiple apps should be represented. Done means producing one MSI for the whole project while preserving the existing application publication behavior.

Written by the indexing model from the issue text.