Allow overrides to be declared in a file (rather than just via the overrides directory)

[Cervator]

As mentioned in #763 (which could very well use a similar strategy as this)

Currently textures can be overridden by putting an identically named texture in an "overrides" directory in a different module. While that's easy it isn't always ideal

Example: LASR holds assets for L&S, which includes a few overrides - yet other modules (like Portals) that has a dependency on LASR (to use the models) thus also get the overridden textures automatically

Instead we could use a module definition file (module.txt?) to map overrides that could be sourced from a dependent module (LASR) rather than the module doing the overriding (L&S)

IMHO it may still be convenient to support the overrides directory as well, akin to the "auto" blockTiles directory - better to eventually use a more comprehensive approach but it works right away for easy progress!

Edit: We may also want to look at module loading order vs. overrides at some point