Build directly to BSA

[focustense]

The file list is known once the asset-copying stage starts, and what isn't known can be computed reasonably efficiently, like the texture paths in mesh files - which extracting is a different operation from copying anyway.

Since the list is known in advance, the file stats (sizes) should also be computable pretty efficiently, and that's enough to fire up the BSA packer and start writing files directly.

It would save on all the staging space and potentially cut build times in half. That's probably being a little optimistic, but I'd expect at least a 25-40% savings on time and a 90% savings on temporary disk space needed. Especially when it comes to slower hard drives, this could be very important.