Add required-by flag for console output

[icanhazstring]

With the rework for version 0.8the required-by flag in the console output is still missing.
This feature needs to be added again so a package is marked as required-by by another package.

This should be done here:
https://github.com/composer-unused/composer-unused/blob/main/src/Console/Command/UnusedCommand.php#L128

If required PackageA is required by a required PackageB, PackageA should get an entry that this package is required-by PackageB even if symbols from PackageA are not used directly in your code.

Example output:

 Used packages
 ✓ php
 ✓ composer-unused/symbol-parser
 ✓ nikic/php-parser (required by composer-unused/symbol-parser, ...)
 ✓ psr/container
 ✓ psr/log