Specialise switch state write functions

[rjbou]

from #6256
The origin of the bug is that for simulated pinning the pinned field in the switch state is not updated. As the switch state is written at the end of pinning process, we don't want to write it.
There is already functions that handle installed package addition our removal, we can have and use only specialised function for switch state writing (update pins, packages, roots), that updates only a subpart of the switch state file.
We can then have a switch state with more information and no more not only update specific fields because they could be written.