Fix loading of preload bundles

Test in mop-integration#22e8d30accb05fa37fba46fd961238bec0a7b29d
Wait for preload to finish before issuing requests for modules

because the module might be included in one of the bundles. This was
already being done to load package descriptions, and so the same
feature has been added to the ScriptLoader
Merge pull request #42 from Stuk/preload

Fix preloading