ModuleFactory should use maps with soft values

As initially suggested on Talk, we should change the static entries in the ModuleFactory class to store the values as SoftReferences rather than the keys, since the keys are almost always Strings, which are not garbage collected, the use of WeakHashMaps here doesn't function as expected. In this case, SoftReferences should be favoured in place of WeakReferences because they are less aggressively GC'ed, meaning they are less likely to be deleted, e.g., during a refresh.