Move PostProcessing submodule to com.unity.postprocessing

.gitmodules

-[submodule "PostProcessing"]
-	path = PostProcessing
-	url = https://github.com/Unity-Technologies/PostProcessing
-	branch = v2
 [submodule "TestProjects/GraphicsTests/Assets/Tests/UTF_Core"]
 	path = TestProjects/GraphicsTests/Assets/Tests/UTF_Core
 	url = https://github.com/Unity-Technologies/UTF_Core.git
+[submodule "com.unity.postprocessing"]
+	path = com.unity.postprocessing
+	url = https://github.com/Unity-Technologies/PostProcessing.git

TestProjects/GraphicsTests/Packages/manifest.json

 {
     "dependencies": {
-        "com.unity.postprocessing": "file:../../../PostProcessing", 
+        "com.unity.postprocessing": "file:../../../com.unity.postprocessing", 
         "com.unity.render-pipelines.core": "file:../../../com.unity.render-pipelines.core", 
         "com.unity.shadergraph": "file:../../../com.unity.shadergraph", 
         "com.unity.render-pipelines.high-definition": "file:../../../com.unity.render-pipelines.high-definition", 

TestProjects/LWRP/GraphicsTests/Packages/manifest.json

 {
     "dependencies": {
-        "com.unity.postprocessing": "file:../../../../PostProcessing", 
+        "com.unity.postprocessing": "file:../../../../com.unity.postprocessing", 
         "com.unity.render-pipelines.core": "file:../../../../com.unity.render-pipelines.core",
         "com.unity.shadergraph": "file:../../../../com.unity.shadergraph",
         "com.unity.render-pipelines.lightweight": "file:../../../../com.unity.render-pipelines.lightweight"