Previously a namespace declaration was imported along with all of its redeclarations, and their members were added to a single Swift extension. This was problematic when a single namespace is declared in multiple modules – the extension belonged to only one of them.
For an example of this, try printing a module interface for `std.string`/`std.iosfwd` – it will be empty, even though the declarations from those modules are actually imported into Swift correctly.
This change makes sure that when we're importing different redeclarations of the same namespace, we're adding them as separate extensions to appropriate modules.
