file-ids are not re-used when there is a collision

While we are detecting collisions we are careful to ensure that file ids from the
official branch are reused, so that we don't get conflicts from random id assignment
if files were added in the upload.

However, when we find a collision we re-import the package and don't share the
file ids and so we can have path conflicts between the new revisions and the squashed
revisions.

Thanks,

James