application: Simplify creation of configuration file finder

This commit simplifies and updates the method for handling the creation
of the configuration file finder.  The main takeaways are:

- Renaming the parameter (i.e., `extra_config_files`) to be a noun to
  better describe the input versus what the option name was called.
- Refreshed the docstring to align with changes from previous commits
  and this one.
- Remove the check to see if it a configuration file finder was already
  created.  This is not necessary as each entire run of a `flake8` calls
  this method once.  When the programmatic API is provided,
  configuration would be provided 'in-code' and not rely upon external
  files/resources.
- Use directly the program name from the `Application`, itself since
  this is the authoritative source for defining it.

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