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/macros/macros.pri

@@ -1,3 +0,0 @@
-include(macros_dependencies.pri)
-
-LIBS *= -l$$qtLibraryTarget(Macros)

src/plugins/macros/macros_dependencies.pri

@@ -1,4 +1,6 @@
-include(../../plugins/coreplugin/coreplugin.pri)
-include(../../plugins/locator/locator.pri)
-include(../../plugins/find/find.pri)
-include(../../plugins/texteditor/texteditor.pri)
+QTC_PLUGIN_NAME = Macros
+QTC_PLUGIN_DEPENDS += \
+    coreplugin \
+    locator \
+    find \
+    texteditor