module: refator ESM loader for adding future synchronous hooks

This lays the foundation for supporting synchronous hooks proposed
in nodejs/loaders#198 for ESM. In addition
this corrects and adds several JSDoc comments for internal functions
of the ESM loader, as well as explaining how require() for
import CJS work in the special resolve/load paths. This doesn't
consolidate it with import in require(esm) yet due to caching
differences, which is left as a TODO.