Describe the bug

@evilebottnawi : It is invalid because we will specify HTML and CSS as entry points in the near future + dependOn, if you have HTML entrypoint and use --output=/path/to/file.js it was wrong, Ideally we should remove --output in favor --output-path, also this one will be consistent with webpack@5 cli args

What is the current behavior?

Passing a file in output flag sets filename object in output

To Reproduce
Pass /path/to/file.js

Expected behavior
It should only set output.path, filename should only be configured using config

Screenshots

Please paste the results of webpack-cli info here, and mention other relevant information

Additional context
#1832 (comment)