Set OpenSceneGraph_${component}_FOUND to TRUE for existing target.

Following statement will never set OpenSceneGraph_osgText_FOUND to TRUE:

```
FIND_PACKAGE(OpenSceneGraph REQUIRED COMPONENTS osgViewer osgText CONFIG)
```

It cause cmake failure, the error message is confusing:
```
Found package configuration file:

/usr/local/osg/gl2/lib/cmake/OpenSceneGraph/OpenSceneGraphConfig.cmake

but it set OpenSceneGraph_FOUND to FALSE so package "OpenSceneGraph" is
considered to be NOT FOUND.
```

I know one doesn't need to add osgText after osgViewer, but it should
not cause an error with confusing message.

packaging/cmake/OpenSceneGraphConfig.cmake.in

@@ -95,6 +95,8 @@ foreach(component OpenThreads osg ${OpenSceneGraph_FIND_COMPONENTS})
 
     if(TARGET @PKG_NAMESPACE@::${component})
         # This component has already been found, so we'll skip it
+        set(OpenSceneGraph_${component}_FOUND TRUE)
+        set(OPENSCENEGRAPH_${component}_FOUND TRUE)
         continue()
     endif()