Need to patch modmaps before every build

When moving swift-xcode projects to a new location, they won't build anymore if they contain modmaps. This is due to the absolute links in them.

We can fix this by running the patcher before every build.

Note: The current user-level fix is to `rm -r .buildzz` after moving a project. This will refetch the image and patch it. If Package.swift already modifications, touch it and rebuild.
