Avoid loading modules recursively.

[typeswitch-dev]

Currently the way modules are loaded is a bit silly: when elaborating module definitions, if an import statement is found, it stops elaborating the current module and starts elaborating the imported module instead (unless it's already loaded). Instead of this, module loading should all happen at a top level, and module imports should be more about making namespaces visible between the various modules loaded at the top level. (Some experimentation is required to figure out how.)