Determine if a directory is a package root of sorts

Finding the root of a package is useful as it defines a point in a code tree where we can summarize things.
This is a complex topic. To help we can:

- look for patterns of key files (as identified with `is_key_file` by the FileClassifier plugin with --classify)
- look for package manifests, package and package-like data (where `package-like` is a new new TBD concept when we do not have a proper way to extract metadata) and the files they contain. This may not yield a root, but identifying a set of files that are for a package may be even better than getting a root.