This PR modifies the build process to use a staging directory for _maps.

• First we copy the original _maps directory tree to the staging directory .staging/_maps.
• Next we copy the _maps directory tree of every modular_starfly module to the staging directory.
• Finally, we remove every file present by name in the modular_starfly/starfly_ships/_maps directory tree.

The build process then checks to see if this staging directory exists, and uses it instead of _maps.

This should let us add/change/remove anything we want in a nice modular way.
One caveat; if you provide identical filenames in different modules, whichever one bash picks last will be the winner.
So try to avoid duplication between modules.