Consider correctness of file overwriting (or not) when overwriting is disabled

[alinsavix]

As discussed on the discord, there's some conditions under which refusing to overwrite files (even when configured not to overwrite) might not be the right behavior -- e.g. when the file has actually changed, or when it's something like metadata (which will generally be specific to a given click of the export button)

This came up when I did an initial export of a WMO as just the raw files, which resulted in metadata without fileNameExternal set for the textures; the metadata didn't get updated when I did a full export, resulting in metadata that no longer matched the export I had done. There's the argument that this is the correct behavior, but there's also the argument that maybe not overwriting things should be restricted to actual game assets and not things like metadata.

There's also the idea that you could checksum the file on disk and the file being generated to determine whether the file should be overwritten, but at that point you've already done enough computation and I/O that it might not be a gain at that point. But worth consideration.