Of note: The rpm/yum thing seems to be coupled, so I put it into one
function that tries the yum helper and uses the rpm path otherwise.
Zypper is already its own thing, so this should only be used for yum
and probably dnf (does that still have the helper?)
Zypper can be dropped, as that already used a separate function in the file.
Apk can just be inlined - it's literally one line for installed and another for all packages.
* Added FreeBSD's pkg to __fish_print_packages
* Portmaster completes on installed packages and ports
* Options list as per fish_generate_completions, needs to be tidied
up further but will suffice for now
