distroot, handle_file_collision(): restore src_uri_dest

... if the file collision cannot be resolved

Also adds some safety checks (distfile name must not be empty and should not
be rename_prefix).
roverlay/depres, rule: temporary fix

do not slot-resolve dep_env with a vmod of '>' or '>='.

This is a temporary fix. A more controllable (configurable) solution will be
added once that "regenerate $list of ebuilds" has been implemented.