Support changes of inlined submodules in the VMR

[premun]

Context

Submodules are inlined into the VMR and there are not git submodules in the VMR anymore. Flowing back these changes is impossible as we don't understand the repos these come from.

We agreed that this is not needed for the initial version of backflow but should be supported in the future.

Milestones

Stage 1

We should implement a build check first that just forbids changes in submodules.

Stage 2

• We should have tooling that would update the VMR to a specific version of a submodule (change the folder and the source-manifest.json etc).
• We'd then need to add a check that validates that changes to submodules correspond with this.
• We need to implement backflowing this change into the product repos (setting the SHA there).
• Changes in nested submodules are still forbidden.