Project files & pk3 packager

[mrwonko]

While working on a mod, the files should not be in a pk3, but loose. PK3 is good for sharing, but the good old file system is better while things are still changing.

So just continue the tradition of putting loose files in the mod folder, but introduce project files to keep track of what's going into a pk3 together.

Then, a packager can create a pk3 release from that.

Support recursive asset dependency resolution to add all necessary files to the archive. Optionally repackage files from other pk3s, retaining attribution. Let mods opt out (and use that for vanilla assets). The goal is not to prevent intentional piracy, just accidental one.

[mrwonko]

Each project will be an fs_game, possibly namespaced? PK3 files can be shared across projects via symlinks, with the original files stored in a repository. This way we can leave the original file names intact while being able to rename the symlinks to customize the load order.