Specify a dependency as a|b to check if either a or b is available as a
command. The script itself must check at runtime, which one is actually
available and use the right one. Helper methods for this are provided in
lib/common.zsh:
- `alternatives a b` aliases a to b, if a is not available
- `alternatives-noparam a b` replaces a with b, if a is not available,
and discards any parameters (e.g. for replacing a formatter with cat)
