always make paths absolute before processing files

[Kanti]

The path is currently only an absolute path if you call rector with a directory like this
rector process pathToFile/.
If you call it with a filename instead, the path will be relative.
rector process pathToFile/File.php.
This makes implementing RectorRules with path usage more difficult than necessary.

fixes: sabbelasichon/typo3-rector#4301

[samsonasik]

Thank you @Kanti

[TomasVotruba]

Thanks 👍