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.

Closes #275