repeating config.load() fails to read default_files

[roharvey]

If you happen to call config.load() twice, the latter fails to read from default_files. This leads to difficult to troubleshoot issues like:

pydantic_core._pydantic_core.ValidationError: 1 validation error for AppConfig
SECRET_KEY
  Field required [type=missing, input_value={}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.8/v/missing

This probably is avoidable by very reasonably saying that we shouldn't be calling config.load() twice. However the docs suggest using config.django_manage() which itself calls config.load(), so in that case at least it is unavoidable.

It looks like the method to check for files loaded data in version 5.0 should be updated.