Consolidate *MutatorContext and ModuleContext into BaseModuleContext

Following 99bdb2ab4fd5168a71a20aecf10611425be47ec4 in build/blueprint,
move more methods into BaseModuleContext.

This reapplies I9f8df94f1ae2b55d3cccf7b9468247f3e7cb2ebd after fixing
missing errors thrown for missing defaults modules when
AllowMissingDependencies == true.

Test: m checkbuild
Test: defaults_test.go
Change-Id: Ia17b2bcbf2bac6889c419b2e73953946f6aa40ad

Android.bp

@@ -83,6 +83,7 @@ bootstrap_go_package {
         "android/config_test.go",
         "android/expand_test.go",
         "android/module_test.go",
+        "android/mutator_test.go",
         "android/namespace_test.go",
         "android/neverallow_test.go",
         "android/onceper_test.go",