system.sw.packages broken with multiple package systems

if there's more than one package system found, all subsequent package systems report packages found from the previous ones.

for example, if package system 'slackpkg' returns packages a, b and c but 'rpm' returns x, y and z, rpm is actually reported as returning a, b, c, x, y, z

this only happens if manager is not specified - that is, information about multiple package management systems is returned in single call.