packaging: isolate invocation of WiX to own function Like we did for Inno, we want to split out the building of Mercurial from invoking the packaging tool so that we can introduce an alternate build mechanism. As part of this refactor, there are inconsequential changes to file layouts. Before, some shared files such as the WiX binaries and merge modules would be installed under build/. Now, they are installed under build/wix-*. This is to keep implementation simpler. But it also helps keep build state more isolated. Differential Revision: https://phab.mercurial-scm.org/D8474