Fix 'runtime-modules.js' pulling in almost all of traceur.

This is a follow up to ab1ad6ab676ae46e71b942050b5f4a617973a42a

Now 'src/runtime/runtime-modules.js' only contains the minimum needed
to handle 'getModuleInstanceByUrl'. This means that, except for
'src/util/url.js', we really can import from standard modules during
'traceur' load.

In addition, as before, compiled code can do the same thing, but now
with the minimum amount of extra runtime weight.