Although the existing require calls would work fine, it's more
consistent to make them actual imports and let them be converted at
compile/bundle time. It also lets us emit ESM if needed.
There are still conditional requires present for things like the node
System, but that's a more difficult problem to solve.
This step converts each file into an exported module by hoisting the namespace bodies into the global scope and transferring internal markers down onto declarations as needed.
The namespaces are reconstructed as "barrel"-style modules, which are identical to the old namespace objects in structure. These reconstructed namespaces are then imported in the newly module-ified files, making existing expressions like "ts." valid.
