use init functions to register module types, etc.

Instead of putting all the blueprint registrations in soong_build,
put them all in init() functions.  This puts the registration next
to the implementation.

Change-Id: Ide1a749518f5e9d1367a18ab3bb1d91da3310c76

common/env.go

@@ -15,6 +15,7 @@
 package common
 
 import (
+	"android/soong"
 	"android/soong/env"
 
 	"github.com/google/blueprint"
@@ -27,6 +28,10 @@ import (
 // compare the contents of the environment variables, rewriting the file if necessary to cause
 // a manifest regeneration.
 
+func init() {
+	soong.RegisterSingletonType("env", EnvSingleton)
+}
+
 func EnvSingleton() blueprint.Singleton {
 	return &envSingleton{}
 }