In the new pipeline the root module is split off in its individual
isolated modules at the end of `isolate`. Before splitting, there are a
lot of global variables in the root module and only a small part is
going to be needed after splitting for each module. To avoid excessive
memory usage employ `ConservativeModuleCloner` in `Isolate` so that
only the needed global variables are actually cloned when splitting off.
Giacomo Vercesi
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