vtkImagingOpenGL2: update to the new module system

Imaging/OpenGL2/CMakeLists.txt

-set(Module_SRCS
-  vtkOpenGLImageGradient.cxx
+set(classes
+  vtkOpenGLImageGradient
   )
 
-vtk_module_library(${vtk-module} ${Module_SRCS})
+vtk_module_add_module(VTK::ImagingOpenGL2
+  CLASSES ${classes})

Imaging/OpenGL2/module.cmake

-vtk_module(vtkImagingOpenGL2
-  BACKEND
-    OpenGL2
-  TEST_DEPENDS
-    vtkTestingRendering
-    vtkInteractionStyle
-  DEPENDS
-    vtkImagingGeneral
-    vtkRenderingOpenGL2
-  PRIVATE_DEPENDS
-    vtkCommonCore
-    vtkCommonDataModel
-    vtkCommonExecutionModel
-    vtkRenderingOpenGL2
-)

Imaging/OpenGL2/vtk.module

+NAME
+  VTK::ImagingOpenGL2
+LIBRARY_NAME
+  vtkImagingOpenGL2
+DEPENDS
+  VTK::ImagingGeneral
+  VTK::RenderingOpenGL2
+PRIVATE_DEPENDS
+  VTK::CommonCore
+  VTK::CommonDataModel
+  VTK::CommonExecutionModel
+  VTK::RenderingOpenGL2
+TEST_DEPENDS
+  VTK::InteractionStyle
+  VTK::IOImage
+  VTK::TestingRendering