E136: A module name can't be qualified by another module

module::function(...) reaches into one module. Modules aren’t nested inside each other, so there is no a::b::function – you have written two module names in a row.

import io;import mem;mem::io::printline("hi");           // `io` is not inside `mem`

Use the one module that actually has the function:

io::printline("hi");

If you meant a type or an enum inside a module, that is the one form with three parts: module::Type::Variant, e.g. shapes::Color::Red. The check only complains when the middle name is itself a module.

Run turf --explain E136 in a terminal to read this again.