Update import paths for changes to blueprint

Blueprint has been modified to include a canonical import path,
update soong to match.

Change-Id: If29d31afbf3bf2e6364961a66456fa5f8f738455
diff --git a/genrule/genrule.go b/genrule/genrule.go
index 6c8d75b..2a90e24 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -17,8 +17,8 @@
 import (
 	"path/filepath"
 
-	"blueprint"
-	"blueprint/pathtools"
+	"github.com/google/blueprint"
+	"github.com/google/blueprint/pathtools"
 
 	"android/soong/common"
 )