A few easy migrations of module code from the runtime to the module map.
The module map already has a few places where it needs a handle scope,
so we're not coupling it any further with the v8 runtime.
`init_runtime_module_map` is replaced with an option to reduce API
surface of JsRuntime.
`module_resolve_callback` now lives in the `ModuleMap` and we use a
annex data to avoid having to go through the `Rc<RefCell<...>>` stored
in the `JsRuntime`'s isolate.
This is a quick first refactoring to split the tests out of runtime and
move runtime-related code to a top-level runtime module.
There will be a followup to refactor imports a bit, but this is the
major change that will most likely conflict with other work and I want
to merge it early.
