Allow migrations to depend on a migration only part of a squashed migration

[Markus Holtermann]

If a migration depends on another migration that is part of a squashed migration (e.g. 6_auto depends on 5_auto which is part of 3_squashed_5) removing the squashed migration (e.g. 5_auto) leads to a KeyError:

Traceback (most recent call last):
  File "/home/markus/Coding/django/django/test/utils.py", line 216, in inner
    return test_func(*args, **kwargs)
  File "/home/markus/Coding/django/tests/migrations/test_loader.py", line 263, in test_loading_squashed_complex2
    loader.build_graph()
  File "/home/markus/Coding/django/django/db/migrations/loader.py", line 233, in build_graph
    self.graph.add_dependency(migration, key, parent)
  File "/home/markus/Coding/django/django/db/migrations/graph.py", line 42, in add_dependency
    raise KeyError("Migration %s dependencies reference nonexistent parent node %r" % (migration, parent))
KeyError: "Migration migrations.6_auto dependencies reference nonexistent parent node ('migrations', '5_auto')"

Thus the whole idea of squashmigrations to be able to eventually remove old migrations is not really working out since a 3rd party app might depend on a specific migration.

I build a testcase: ​https://github.com/Markush2010/django/commit/9a983d4fe66c583858eb370497d0b66227690f79