support multiple package paths in ARGOS_PACKAGES_DIR

[milahu]

this would be helpful to use a system-wide package manager like apt or nix
to install argos packages into a system-wide folder like /usr/share/argos-translate/packages

argospm installs packages to $HOME/.local/share/argos-translate/packages

argos-translate can use a different packages path via the env var ARGOS_PACKAGES_DIR
ideally, argos-translate should accept multiple paths separated by colon, for example
/usr/share/argos-translate/packages:$HOME/.local/share/argos-translate/packages
the system-wide folder will be different on different linux distributions

downstream issue: NixOS/nixpkgs#250863 Packaging request: Argos translation data as Nix packages

It would be nicer to handle this through Nix as well,
similarly Aspell and Hunspell dictionaries which are packaged in Nixpkgs.

The only sane way to achieve this it to add support for this use-case to the upstream project,
similarly to how whisper and whisper.cpp handles data files.