Code review comment for lp://staging/~zematynnad/rnr-server/architecture_688121

I think as well as adding arch_tag as an optional argument to lp_verify_packagename_in_distro, you'd need to pass arch_tag in when you call the function, in forms.py (probably also adding the arch verification to the check in _validate_and_populate_software_item); could you check this?