Put lib/vendor close to the webapp

The files have nothing to do with binaries on the system but are only
relevant for executable code from DOMjudge (which has no clearly defined
place in FHS).

This was discussed in a private channel with the maintainers:
https://domjudge-org.slack.com/archives/GJ2JX7B7X/p1709760201078489?thread_ts=1709540681.476949&cid=GJ2JX7B7X

The configure file is not cleaned up yet to clearly show this choice in
the next minor release. If there is no new discussion this can be
changed in the next major release.

Author: vmcj

configure.ac

@@ -170,7 +170,7 @@ if test "x$FHS_ENABLED" = xyes ; then
 	AC_SUBST(domserver_webappdir,       $datadir/${PACKAGE_TARNAME}/webapp)
 	AC_SUBST(domserver_sqldir,          $datadir/${PACKAGE_TARNAME}/sql)
 	AC_SUBST(domserver_libdir,          $libdir/${PACKAGE_TARNAME})
-	AC_SUBST(domserver_libvendordir,    $libdir/${PACKAGE_TARNAME}/vendor)
+	AC_SUBST(domserver_libvendordir,    $datadir/${PACKAGE_TARNAME}/lib/vendor)
 	AC_SUBST(domserver_logdir,          $localstatedir/log/${PACKAGE_TARNAME})
 	AC_SUBST(domserver_rundir,          $localstatedir/run/${PACKAGE_TARNAME})
 	AC_SUBST(domserver_tmpdir,          /tmp)