Support importing functions from other module namespaces

[chrisdickinson]

The extism-js cli today flattens all typescript interfaces other than main into a single, flat import namespace. We may want to retain the structure of those other interfaces: consider two interfaces, foo:bar/baz, and apple:banana/cantaloupe, that both export frobnicate. We want to be able to call both foo:bar/baz.frobnicate and apple:banana/cantaloupe.frobnicate from our module.