Move import to make it less confusing

An imported function, _parse_pkginfo(), requires the collections module.
It has been removed a couple times, causing regressions each time. Since
it is needed in just one place, this commit moves it to where it is
needed and includes a warning not to remove the import.

See https://github.com/saltstack/salt/pull/7017#issuecomment-23757320
for more info.