Code review comment for lp://staging/~paelzer/curtin/bug-1574113-derived-repositories

further reading and thinking.
I think that the implementation doens't actually solve the thing we need solved.

apt_sources configuration to curtin declares what the target's apt should be configured with (much the same as the storage_config or 'kernel' configuration is about the installed environment not the host).

So, the apt_sources needs to apply to the target.
For maas, to get all of this working together, it has to
a.) declare to cloud-init that certain apt sources are necessary in the ephemeral environment
b.) declare to curtin that the target environment must include apt configured with a set of apt sources.

'a' is done in cloud-init user-data
'b' is done in curtin config.