Allow extends-clauses in package.order.

[HansOlsson]

Closes #3438

The reasons for not making it more required are:

• For backwards compatibility reasons we should be able to read packages where "package.order" doesn't include extends.
• If extends-clauses must be included in "package.order" tools would anyway need a compatibility setting when working with libraries using an older version.
  Thus it makes sense that we only add them when needed.

[henrikt-ma]

It is not strong enough to say should. It needs to be:

…, but must be omitted if…

Also, I think "normal classes" can be explained more clearly as something like non-extends entries in the package.order.

[henrikt-ma]

I think the entire paragraph needs to be rewritten with extends-clauses in mind, and why not deprecate the case of a stray extends-clauses?:

In order to preserve the order of sub-entities it is advisable to create a file \filename{package.order} where each line contains the name of one class or constant (in Modelica \lstinline!IDENT! form), or \lstinline!extends!-clause (in the form \lstinline!extends A.B! -- one space, no semicolon).
An \lstinline!extends!-clause entry shall be omitted from \filename{package.order} if it would appear before any non-\lstinline!extends!-clause entry.
If a \filename{package.order} is present when reading a structured entity, the classes, constants, and base classes are added in this order, following the non-listed base classes; if the \filename{package.order} content does not exactly match the package content, the resulting order is tool specific and a warning may be given.
A deprecated behavior is that an \lstinline!extends!-clause may be omitted anywhere in the \filename{package.order}.
The relative order of classes, constants, and \lstinline!extends!-clauses that are stored in \filename{package.mo} must be consistent with the relative order of the corresponding entries in \filename{package.order} (that is, the \filename{package.order} only adds information regarding the ordering of classes not stored in \filename{package.mo}).

[HansOlsson]

To illustrate:

package P
  constant Real p=2;
  extends PBase;
  constant Real q=2;
  model B end B;
end P;

Language group: can find order of PBase even without package.order.