`is_manifest` flag is not accurately determined

[AyanSinhaMahapatra]

See

scancode-toolkit/src/summarycode/classify.py

Line 42 in c326a0b

_MANIFEST_ENDS = {

and set_classification_flags where we compute the is_manifest flag (for --classify) based on a fixed list which is outdated and a small subset of all the package manifests we detect.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

scancode-toolkit/src/packagedcode/__init__.py

Line 52 in c326a0b

APPLICATION_PACKAGE_DATAFILE_HANDLERS = [

and maintain a dymanic list based on all supported manifests.