goose: strip vendor from generated import paths

This allows goose to work more gracefully in a vgo setting.

Reviewed-by: Tuo Shan <shantuo@google.com>

internal/goose/testdata/Vendor/foo/foo.go

@@ -0,0 +1,7 @@
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println(injectedMessage())
+}