Consider detected copyrights when determining a declared holder from a package manifest in summary plugin

When scanning the package `atheris v 2.0.11` (https://github.com/google/atheris/archive/refs/tags/2.0.11.tar.gz) using the `--summary` plugin, the `declared_holder` value in the scan summary is `Bitshift`, which is the author of the package. This was determined from the parsed package data from the `setup.py` file of `atheris`. However, the `setup.py` contains a comment that is a copyright statement with the actual copyright holders. The summary plugin should be updated to also consider copyrights detected by the copyright scanner. This value should take precedence over authors.