Qualified name using a nested namespace

[jiribenes]

Currently, one needs to write mutable::array::fill for a function fill imported from module mutable/array. I'd like to write just array::fill instead :) https://github.com/effekt-lang/effekt/blob/c93e5c695553e7f8958241bea8588a435baf9429/examples/pos/arrays.effekt#L22