Overrides to `packages.*.source` aren't working

[aakropotkin]

This is a "maybe aren't working".

I noticed when created a new test case that foverrides.nix containing config.floco.packages."@floco/phony"."4.2.0".source = ...; wasn't being used by prepared. I tried this with mkForce and tried splitting the declaration into "4.2.0" = { source = ...; } and didn't have any luck.

Investigate to see if these overrides still work as expected and add a test case.