[cpp.pre][cpp.module] Consolidate preprocessor grammar in one clause

The grammar around module directives is quite precise and involved,
yet spread across two clauses, the preamble and the module directive
clauses.  This change set consolidates all the grannar for a _module-file_
into the module directive clause, making it easier to grasp all at once.
None of these terms are referenced from outside the clause.

Author: AlisdairM

source/preprocessor.tex

@@ -16,21 +16,6 @@
     module-file
 \end{bnf}
 
-\begin{bnf}
-\nontermdef{module-file}\br
-    \opt{pp-global-module-fragment} pp-module \opt{group} \opt{pp-private-module-fragment}
-\end{bnf}
-
-\begin{bnf}
-\nontermdef{pp-global-module-fragment}\br
-    \keyword{module} \terminal{;} new-line \opt{group}
-\end{bnf}
-
-\begin{bnf}
-\nontermdef{pp-private-module-fragment}\br
-    \keyword{module} \terminal{:} \keyword{private} \terminal{;} new-line \opt{group}
-\end{bnf}
-
 \begin{bnf}
 \nontermdef{group}\br
     group-part\br
@@ -1214,6 +1199,21 @@
 \rSec1[cpp.module]{Module directive}
 \indextext{preprocessing directive!module}%
 
+\begin{bnf}
+\nontermdef{module-file}\br
+    \opt{pp-global-module-fragment} pp-module \opt{group} \opt{pp-private-module-fragment}
+\end{bnf}
+
+\begin{bnf}
+\nontermdef{pp-global-module-fragment}\br
+    \keyword{module} \terminal{;} new-line \opt{group}
+\end{bnf}
+
+\begin{bnf}
+\nontermdef{pp-private-module-fragment}\br
+    \keyword{module} \terminal{:} \keyword{private} \terminal{;} new-line \opt{group}
+\end{bnf}
+
 \begin{bnf}
 \nontermdef{pp-module}\br
     \opt{\keyword{export}} \keyword{module} \opt{pp-tokens} \terminal{;} new-line