make library and plugin dependencies declarative

instead of directly including the respective pri files in
*_dependencies.pri, set variables and let qtcreator.pri resolve
them to includes.
this will allow us to re-use the dependency info elsewhere.

Change-Id: Iaa33924e428ac0409660f42df2f98a7978452d3e
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/plugins/qmlprojectmanager/qmlprojectmanager.pri

@@ -1,2 +0,0 @@
-include(qmlprojectmanager_dependencies.pri)
-LIBS *= -l$$qtLibraryName(QmlProjectManager)

src/plugins/qmlprojectmanager/qmlprojectmanager_dependencies.pri

@@ -1,6 +1,8 @@
-include(../../plugins/projectexplorer/projectexplorer.pri)
-include(../../plugins/texteditor/texteditor.pri)
-include(../../plugins/qmljseditor/qmljseditor.pri)
-include(../../plugins/qmljstools/qmljstools.pri)
-include(../../plugins/debugger/debugger.pri)
-include(../../plugins/qtsupport/qtsupport.pri)
+QTC_PLUGIN_NAME = QmlProjectManager
+QTC_PLUGIN_DEPENDS += \
+    projectexplorer \
+    texteditor \
+    qmljseditor \
+    qmljstools \
+    debugger \
+    qtsupport