Feature request - allow to define subcollections in different files from the parent collection

[liranzairi]

In our project we have a collection that has quite a few sub-collections, some of them also have quite a few sub-collections, etc. Today all this collection subtree has to be defined in the same file, at most it can be separated using the "part" syntax. In addition to a reduced quality of module separation, this also slows down the build_runner cycle and every small change to one of the classes in that subtree causes a long build cycle (~3 minutes).

Is there any chance to support defining sub-collections in different files than the parent collection?