6287: Don't replace entire module and file nodes when inserting imports r=matklad a=Veykril
This change minifies the resulting diff of import insertions by inserting or replacing the produced use tree directly through an `action` return value instead replacing the entire container node. This action has to be applied by the caller now. This unfortunately pulls the `AssistBuilder` into scope of `insert_use` back again but I tried to at least keep it away from the `insert_use` fn itself.
I'm open to more/better ideas regarding this :)
Fixes#6196
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
060c8b2c96