Use 'depends' directory

[Z-snails]

This is designed to provide idris2 with local dependencies, ideal for Inigo.
This will hopefully also allow building packages without a Inigo.toml file (or maybe Inigo.dhall soon!)

Currently there's no way to set this from the .ipkg file, but I'll see if that can be changed.

[Z-snails]

Actually, I'm not sure how this will interact with dependecies that have their own dependencies, but it's definitely worth a thought!