What you're describing is categorization. It's not the same thing as an hierarchy - using a hierarchy for categorization limits you to one category only, which is often insufficient. E.g. XML is both a standard and a format that can be parsed, and we can't put it both in Parser.pmod and Standards.pmod (or in an equally relevant Formatter.pmod).
But a worse side-effect from using container modules to improve the documentation is that the names actually used in the code also have to carry around these categories. True, there's the import directive to help in such cases, but it brings its own set of problems.
