`is_manifest` flag is not accurately determined

See https://github.com/aboutcode-org/scancode-toolkit/blob/c326a0b6ed7dda44e40c8a436c7005672a1a53a9/src/summarycode/classify.py#L42 and `set_classification_flags` where we compute the `is_manifest` flag (for `--classify`) based on a fixed list which is outdated and a small subset of all the package manifests we detect. We should somehow parse all the `path_patterns` attributes of DataFileHandler classes at https://github.com/aboutcode-org/scancode-toolkit/blob/c326a0b6ed7dda44e40c8a436c7005672a1a53a9/src/packagedcode/__init__.py#L52 and maintain a dymanic list based on all supported manifests.