qt6-build.eclass: update comment for qt-utils, re-order vars a bit

*Was* going to switch this to qt-utils.eclass but after starting
to change variables it felt rather messy.

e.g. QT6_PREFIX, LIBDIR, and SYSCONFDIR do not use the eclass
(there is no 'qt6' in these, but we still need the path), and
then some do not have their own function like examples, imports,
and translations and feel it's not really worth adding them if
nothing else needs it.

Seeing the mixed definitions made me think it was better as-is
and less error-prone.

Alternatively could remove prepare_env entirely and have each
ebuilds use qt-utils directly, but it can be repeated a lot
sometimes (+${EPREFIX} which we always need the way it is used)
and variables setup the way we need them felt nicer and opens
up less subshells while at it.

While here, re-order variables a bit -- the group without "qt6"
in them, the top level DATADIR group referenced by the others,
then the rest.

Signed-off-by: Ionen Wolkens <ionen@gentoo.org>

eclass/qt6-build.eclass

@@ -276,12 +276,14 @@ _qt6-build_create_user_facing_links() {
 # @DESCRIPTION:
 # Prepares the environment for building Qt.
 _qt6-build_prepare_env() {
-	# setup installation directories
-	# note: keep paths in sync with qmake-utils.eclass
+	# setup variables for installation directories
+	# note: qt-utils.eclass should be kept in sync, eclass is not used
+	# here for clarity and for paths that are only defined here
 	QT6_PREFIX=${EPREFIX}/usr
-	QT6_DATADIR=${QT6_PREFIX}/share/qt6
 	QT6_LIBDIR=${QT6_PREFIX}/$(get_libdir)
+	QT6_SYSCONFDIR=${EPREFIX}/etc/xdg
 
+	QT6_DATADIR=${QT6_PREFIX}/share/qt6
 	QT6_ARCHDATADIR=${QT6_LIBDIR}/qt6
 
 	QT6_BINDIR=${QT6_ARCHDATADIR}/bin
@@ -293,7 +295,6 @@ _qt6-build_prepare_env() {
 	QT6_MKSPECSDIR=${QT6_ARCHDATADIR}/mkspecs
 	QT6_PLUGINDIR=${QT6_ARCHDATADIR}/plugins
 	QT6_QMLDIR=${QT6_ARCHDATADIR}/qml
-	QT6_SYSCONFDIR=${EPREFIX}/etc/xdg
 	QT6_TRANSLATIONDIR=${QT6_DATADIR}/translations
 }