wix: add a hook for a prebuild script to inject extra libraries I need this to build packages for Google so we can bundle some extensions in the installed image. My assumption is that this is most interesting for the .msi images so I only wired it up there. I'm not thrilled with the interface this provides, but it was an easy way to retain debug messages on Windows while also having enough structure to know what lines are actually module names for py2exe. Still pending on my end: I need to bundle a couple of config files, and at least one data file. I'm open to advice on how to do those things, and how to do this better. Differential Revision: https://phab.mercurial-scm.org/D6164