Split _fish_deps_pm.fish into one file per function so Fish can
autoload each by name (_fish_deps_detect_pm, _fish_deps_pm_install,
_fish_deps_pm_upgrade). A single file with multiple functions only
autoloads under the filename, leaving the others unreachable.
Replace all `test -a`/`test -o` binary operators with `; and`/`; or`
chains throughout _fish_deps_install and _fish_deps_update. Fish 4.x
removed support for -a/-o in test, causing the "unexpected argument"
errors seen on Debian.
Also consolidate paru/yay cases in pm_install and pm_upgrade since
both helpers use identical flags.
