appimage rule fails on directory dependencies

[gtpalmer]

If the binary one is trying to build an appimage from contains a directory dependency, for example a filegroup, the build will fail with:

...line 264, in copyfile
    with open(src, 'rb') as fsrc, open(dst, 'wb') as fdst:
IsADirectoryError: [Errno 21] Is a directory: <directory target path>