Improve presentation of file system mapping

[henrikt-ma]

This is a follow-up to #3885 with general cleanup of the the presentation in File System Mapping of Package/Class.

Improvements include:

• More careful introduction and use of terminology (structured entity and nonstructured entity).
• Aligned language in subsections to make similarities easier to spot.
• Add missing rule that package.mo should only define a single class.
• Expand non-normative section on sub-directories without package.mo.