Preemptively determine the path of the user configuration file during
the construction of the `ConfigFileFinder` object.  The user
configuration path will always be the same, regardless of when it gets
obtained by a run of `flake8`.

This isolates the logic of determining the user configuration path into
a static helper method to be called to set the `.user_config_file`
attribute.  The helper method leverages `utils.is_windows()`, instead of
reimplementing the check, and decomposes clearly the directory name and
the base name to construct the path with a single `return` path.
Additionally, this avoids reconstructing the path on demand of obtaining
the user configuration file path.

Remove the checks to see if a configuration file has already been seen
and parsed.  These checks aren't necessary because the entire run of
`flake8` calls these methods *only* once per configuration provided.

Ensure that a not-found file produces an error

Closes #600

See merge request pycqa/flake8!404

split lines the same when read from stdin

Closes #270

See merge request pycqa/flake8!406

- this restores a flake8 2.x behaviour that was lost in the refactor

This simplifies the number of required parameters needed for the
`ConfigFileFinder` object throughout the various tests.

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.