get rid of custom 'qt' gitmodules property

instead, re-use the (also custom) 'project' property with the magic
value '-' to flag modules which contain no project files at all.

Change-Id: I33163c098852e1b79ad698115aac98217efe1157
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Aapo Keskimolo <aapo.keskimolo@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Iikka Eklund <iikka.eklund@qt.io>
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

.gitmodules

@@ -68,7 +68,7 @@
 	url = ../qtrepotools.git
 	branch = master
 	status = essential
-	qt = false
+	project = -
 [submodule "qtwebkit"]
 	depends = qtbase
 	recommends = qtdeclarative qtlocation qtmultimedia qtsensors qtwebchannel qtxmlpatterns

qt.pro

@@ -29,7 +29,8 @@ QMAKE_INTERNAL_INCLUDED_FILES += $$PWD/.gitmodules
 modules = $$sort_depends(modules, module., .depends .recommends .serialize)
 modules = $$reverse(modules)
 for (mod, modules) {
-    equals(module.$${mod}.qt, false): \
+    project = $$eval(module.$${mod}.project)
+    equals(project, -): \
         next()
 
     deps = $$eval(module.$${mod}.depends)
@@ -43,7 +44,6 @@ for (mod, modules) {
     !isEmpty(QT_BUILD_MODULES):!contains(QT_BUILD_MODULES, $$mod): \
         next()
 
-    project = $$eval(module.$${mod}.project)
     isEmpty(project) {
         !exists($$mod/$${mod}.pro): \
             next()