Soong support for namespaces

Bug: 65683273
Test: build/soong/scripts/diff_build_graphs.sh \
      --products=aosp_arm \
      'build/blueprint:work^ build/soong:work^' \
      'build/blueprint:work build/soong:work'
      # and see that the only changes were:
      # 1. adding some new files
      # 2. changing some line numbers
Test: m -j nothing # which runs unit tests

Change-Id: I6d3e52ef62c4cabe85b9a135a54de0e1a6aab29c

Android.bp

@@ -50,6 +50,7 @@ bootstrap_go_package {
         "android/makevars.go",
         "android/module.go",
         "android/mutator.go",
+        "android/namespace.go",
         "android/onceper.go",
         "android/package_ctx.go",
         "android/paths.go",
@@ -67,6 +68,7 @@ bootstrap_go_package {
     testSrcs: [
         "android/config_test.go",
         "android/expand_test.go",
+        "android/namespace_test.go",
         "android/paths_test.go",
         "android/prebuilt_test.go",
         "android/util_test.go",