Exhaustive modules graph traversal (#68865)
### What?
Improves the traversal of the module graph by doing a more exhaustive search of the children modules, thus increasing the "hit rate" for optimized module IDs (which should be 100%).
### Why?
Some structs that implement the `Module` trait return an empty list in the `references` method, but still "own" some child modules. This is the case of `EcmascriptClientReferenceModule`, for example, which returns empty in `references` but owns a `client_module` and an `ssr_module`. These modules are ignored when traversing the module graph with the `get_referenced_modules` function we used before, so neither they nor their children are ever assigned an optimized ID.
### How?
To solve this, a `children_modules` method is added to the `Module` trait, which should exhaustively return all direct children of the module. The function defaults to `primary_referenced_modules`, which simply calls `references`, but structs like `EcmascriptClientReferenceModule` can reimplement it and return "hidden" children modules like the aforementioned `client_module` and `ssr_module`.
