Moved ReducedGraphParent building to a new module
Cleanup unused imports resulting from move of reduced graph building
Purged ReducedGraphParent
Removed a bunch of unnecessary .clone()s from build_reduced_graph
auto merge of #20115 : gereeter/rust/split-module-building, r=eddyb

This also gets rid of a bunch of unnecessary `.clone`s.

cc @eddyb