Warn when packing a target that has different versions of the same file

[squattingmonk]

When unpack finds multiple copies of a source file, it asks the user which one to replace. However, the pack loop does not, which can result in the user scratching their head trying to find out why the packed version of a module doesn't match their source tree. nasher should warn the user when duplicate files are found or (ideally) ask which one to use.

Source: Discord thread