Now that `ConfigFileFinder.config_file` attribute is used everywhere and
is constructed from the `--config` CLI option, the now unused
`cli_config` parameters can be safely removed.

Now that `ConfigFileFinder.ignore_config_files` attribute is used
everywhere and is constructed from the `--isolated` CLI option, the now
unused `isolated` parameters can be safely removed.

This fixes a regression introduced in !346 to ensure that `--config` and
`--isolated` are recognized in `aggregate_options()`.

The regression manifested because `aggregate_options()` was relying on
re-parsing `argv` to obtain the option values.  However, !346 changed
the preliminary parsing logic to only parse and "eat" what is necessary
and forward along the options needed before all the configuration was
loaded.  This code path was overlooked because the tests in
`test_aggregator()` were passing but the call from the `Application`
object would never have these options in the remaining `argv` list to be
passed long.

Simplify the initialization code path by invoking
`config.ConfigFileFinder` directly instead of the extra hop by calling
the static class-level `Application.make_config_finder()` method.

This change removes the `.config_finder` object from the `Application`.
Since the configuration finder is only needed during initialization, we
constrain the finder to be returned and passed to other methods
necessary for initialization.

This change makes `Application.make_config_finder` to be side-effect
free where it its return value is only determined by its input values.

Positional arguments are not used nor parsed for pre-configuration
loading.  Thus, renaming the method and updating the docstrings
appropriately.