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.
Fixes some potentially unsafe uses of direct substitution into regex
expressions and also switches some completions to regex-based now that
there is a safe way of using it.
Use `apt-cache show` instead of `apt-cache packagenames` to efficiently
print package names and a brief description instead of the placeholder
(localized) "Package" text that was previously printed. This applies to
both available and installed packages (for inistall and remove operations,
respectively).
TODO: update `__fish_print_packages` for non-debian platforms to do the
same.
Added a new flag `--installed` via `argparse` to `__fish_print_packages`
which indicates that only installed packages should be listed.
TODO: Other non-debian/apt platforms should take advantage of this flag/
behavior as well.
