Add visibility support

Implementation uploaded for review. Includes unit tests but does not
yet handle prebuilts, that will come in a future change once some
more general issues with prebuilts and namespaces is resolved.

See README.md#Visibility for details of what this does and how to use
it.

Bug: 112158820
Test: add visibility rules for core library modules, make core-tests
Change-Id: I8ec980554398ad6f2d42043ce518f811a35da679

Android.bp

@@ -70,6 +70,7 @@ bootstrap_go_package {
         "android/testing.go",
         "android/util.go",
         "android/variable.go",
+        "android/visibility.go",
         "android/vts_config.go",
         "android/writedocs.go",
 
@@ -90,6 +91,7 @@ bootstrap_go_package {
         "android/rule_builder_test.go",
         "android/util_test.go",
         "android/variable_test.go",
+        "android/visibility_test.go",
         "android/vts_config_test.go",
     ],
 }