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.
