Fix loading main_config again during discovery of dynamic configurations

[cimnine]

Related Issue: #445

New Behavior

The main configuration files, configuration.py and ldap_config.py, were accidentally loaded twice: The initial load was hard-coded and correct. But during the discovery of the dynamic, user-contributed configuration files (like extra.py or whatever.py), the main configuration files were discovered as well and loaded again.
They have thereby overwritten all settings in files that were loaded before them.

An example, given the file a_config.py:

# a_config.py
BANNER_LOGIN = "Hello Mr. Anderson. 💊"

The old behavior was:

1. Load configuration.py
2. Discover a_config.py and configuration.py
3. Set the value of BANNER_LOGIN to what is found in a_config.py.
4. Set the value of BANNER_LOGIN to what is found in configuration.py,
   because it's alphabetically after a_config.py and falsely discovered again.
5. Done.

The new behavior is – as it should be:

1. Load configuration.py
2. Discover a_config.py and configuration.py, but ignore configuration.py, as it's the main config file.
3. Set the value of BANNER_LOGIN to what is found in a_config.py.
4. Done.

Thanks to @tobiasge for discovering this bug.

Contrast to Current Behavior

The described bug has now been fixed. During the discovery for dynamic configurations,
when loading each of the discovered configuration files, the main_config file is now excluded.

Discussion: Benefits and Drawbacks

The intended behavior is achieved: All values of the main configuration files can be overwritten with values in arbitrarily named custom configuration files.

Changes to the Wiki

n/a

Proposed Release Note Entry

## Fix Custom Configuration Discovery #450

There was a bug which prevented that certain custom configuration files were not loaded.
This was the case, when the custom configuration file's name was alphabetically before the main configuration file's name.
I.e. `a_configuration.py` is alphabetically before `configuration.py`. With this file, `a_configuation.py`, it was therefore not possible to overwrite values which are already declared in `configuration.py`. (The same holds true for `ldap_config.py`, but it's worse, because the letter L is relatively late in the alphabet.)

See #450 for more details.

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