printed manuals: xrefs in and out of "Preparing Patches"

Fix two cases of links where the on-line manual is one document but the
manual is split into separate documents for printing:

* doc/emacs/package.texi (Fetching Package Sources): fix printed link to
"Preparing Patches" to point to separate document.
* doc/emacs/vc1-xtra.texi (Preparing Patches): fix printed link to
"Directory Variables" to point to separate document.

doc/emacs/package.texi

@@ -654,7 +654,13 @@ maintainer, you can use the command @code{package-report-bug} to
 report a bug via Email.  This report will include all the user options
 that you have customized.  If you have made a change you wish to share
 with the maintainers, first commit your changes then use the command
-@code{package-vc-prepare-patch} to share it.  @xref{Preparing Patches}.
+@code{package-vc-prepare-patch} to share it.
+@iftex
+@xref{Preparing Patches,,,emacs-xtra, Specialized Emacs Features}.
+@end iftex
+@ifnottex
+@xref{Preparing Patches}.
+@end ifnottex
 
 @findex package-vc-install-from-checkout
 @findex package-vc-rebuild

doc/emacs/vc1-xtra.texi

@@ -312,7 +312,14 @@ If you expect to contribute patches on a regular basis, you can set
 the user option @code{vc-default-patch-addressee} to the address(es)
 you wish to use.  This will be used as the default value when invoking
 @code{vc-prepare-patch}.  Project maintainers may consider setting
-this as a directory local variable (@pxref{Directory Variables}).
+this as a directory local variable
+@iftex
+(@pxref{Directory Variables,,Per-Directory Local Variables,
+emacs, the Emacs Manual}).
+@end iftex
+@ifnottex
+(@pxref{Directory Variables}).
+@end ifnottex
 
 @node Customizing VC
 @subsection Customizing VC