Fix optional-dependencies in mkPythonEditablePackage

The problem is that optional-dependencies looks something like:

    optional-dependencies = {
        extra1 = [ package1 package2 ];
    };

And the goal is to transform that into some JSON that works as a
pyproject file. So, its supposed to look something like:

    {
        "optional-dependencies": {
            "extra1": [ "package1" "package2" ]
        }
    }

Previously, we were trying to do that converstion with:

    optional-dependencies = lib.mapAttrs (_: lib.getName) optional-dependencies;

The problem is that we're calling lib.getName on the array of
dependencies. Instead, we convert the code to call lib.getName on each
individual dependency in the array.