Clarify that only sub-directories containing package.mo are considered for conflict

[HansOlsson]

Closes #3882

[henrikt-ma]

I miss the normative change that makes it clear that a directory only qualifies as an entry if it contains package.mo (as opposed to qualifying by only being a directory, and than having to fulfill the requirement to contain package.mo).

That is, replace this:

The nature of such an external entity falls into one of the following two groups:
\begin{itemize}
\item
  Directory in the file system.
\end{itemize}

\begin{itemize}
\item
  File in the file system.
\end{itemize}

by this:

The nature of such an external entity in the file system falls into one of the following two groups:
\begin{itemize}
\item
  A directory containing a file named \filename{package.mo}.
\item
  A file with filename extension \filename{mo}.
\end{itemize}

[henrikt-ma]

We need normative rules outside the example, and no rules should be given in the example.

[henrikt-ma]

Sure, I'll try to take it from here in a separate PR that is not urgent to merge.