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/git/git_dependencies.pri

@@ -1,5 +1,8 @@
-include(../../plugins/projectexplorer/projectexplorer.pri)
-include(../../plugins/texteditor/texteditor.pri)
-include(../../plugins/coreplugin/coreplugin.pri)
-include(../../plugins/vcsbase/vcsbase.pri)
-include(../../libs/utils/utils.pri)
+QTC_PLUGIN_NAME = Git
+QTC_LIB_DEPENDS += \
+    utils
+QTC_PLUGIN_DEPENDS += \
+    projectexplorer \
+    texteditor \
+    coreplugin \
+    vcsbase