Support (local) downstream cnd override from within an site extension project

When an site extension project needs to update an existing core project owned cnd (.e.g. using the core project namespace), it currently will fail to (re)load after restart.

The problem is that when the site extension project (first) changes/updates the namespace, the updated cnd gets auto-exported (correctly) to the 'upsteam' repository-data/application module, but a restart thereafter against an existing (local) repository therafter will may fail to load the core project cnd because of incompatible changes.

Eventually the site extension project will (have to) upstream their namespace changes to the core project, but until that is done, local development is broken.

One solution might be to allow skipping the core project cnd during restart and automatically (only) load the site extension project replacement cnd instead.
Practically this means replacing/overriding namespaces provided by downstream modules before applying them.