Allow double squashing of migrations

[Markus Holtermann]

While #23090 added a safeguard to prevent squashmigrations from blowing up when squashing an already squashed migration, this should be possible in the future to ease transition for 3rd party projects.

It is generally possible to add a squashed migration as a dependency together with its replaced migrations (e.g. have 0001_initial, 0002_second and 0001_squashed_0002_second in the dependency list). An example can be found here: ​https://github.com/MarkusH/django-ticket-triage/tree/0aac03c597c744909db60772b99c34c99ed7111d/app_a/migrations

The tasks I identified so far are for now:

1. Remove the safeguard
2. When squashing 0001_0002 and 0003, the squashed migration 0001_0003 doesn't consider the already squashed migration operations and doesn't add them to it's operations list
3. Add squashed migrations to the dependency list

[Bruno Alla]

Deassigning as I'm no longer actively working on this.

[Raphael Gaschignard]

I have a potential patch for this one that I'm pretty satisfied with, so self-assigning this one.

[Raphael Gaschignard]

​PR

[Raphael Gaschignard]

Unchecked all the "Needs..." to request review on the implementation in principle.