migrate command's database checks may result in RecursionError

[Tim Graham]

Following the steps in #26488 results in the InvalidBasesError error described in that ticket until 0d3c616fbb2f49fa7ff6809e5a6777275352b35b where something like this happens:

  File "/home/tim/code/django/django/core/management/commands/migrate.py", line 62, in _run_checks
    issues.extend(super(Command, self).check(**kwargs))
  File "/home/tim/code/django/django/core/management/base.py", line 385, in check
    include_deployment_checks=include_deployment_checks,
  File "/home/tim/code/django/django/core/management/commands/migrate.py", line 61, in _run_checks
    issues = run_checks(tags=[Tags.database])
  File "/home/tim/code/django/django/core/checks/registry.py", line 81, in run_checks
    new_errors = check(app_configs=app_configs)
  File "/home/tim/code/django/django/core/checks/database.py", line 9, in check_database_backends
    for conn in connections.all():
  File "/home/tim/code/django/django/db/utils.py", line 226, in all
    return [self[alias] for alias in self]
RecursionError: maximum recursion depth exceeded

The loss of a helpful message isn't desirable.

[Tim Graham]

Duplicate of #26492