Refine the requirements on how RS must process the container directory structure #1687

The EPUB core spec says in the File and Directory Structure section:

EPUB Creators MAY locate all other files within the OCF Abstract Container in any location descendant from the Root Directory, provided they are not within the META-INF directory.

But @mattgarrish reports in #1374 that some reading systems do not handle that correctly. They don't allow content that is not in the directory of the Package Document (or any descendant directory).

There are at least two (exclusive) options:

• add language in the RS spec to say more explicitly that RS must be able to handle all container content.
• add restrictions to the Core spec that say authors should not rely on content that is not in the Package Document directory and descendants.