Abstract the creation of a default composer.json to `create_default_composer_json()`
This error message shouldn't never be needed.

`$json_file->write( $options );` will throw an Exception if it runs into an error. We can safely depend on this.
More abstraction: `find_composer_path()` determines where composer.json should be
More suggestive method name
Support for WP-CLI installs that are straight git clones