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/common/arch.go b/common/arch.go
index cb4e998..8cc3b10 100644
--- a/common/arch.go
+++ b/common/arch.go
@@ -15,8 +15,8 @@
 package common
 
 import (
-	"blueprint"
-	"blueprint/proptools"
+	"github.com/google/blueprint"
+	"github.com/google/blueprint/proptools"
 	"fmt"
 	"reflect"
 	"runtime"