support per-package visibility

this should support BUILD level and package level visibilities:

• package(default_visibility = ["//visibility:public"]) (https://github.com/ApolloAuto/apollo/blob/d798f5529d2001f807e6eb1fdc37cb74e3dddc36/third_party/boost.BUILD#L3) can be supported via a config in the bazelize plugin

• visibility = ["//visibility:public"], (https://github.com/ApolloAuto/apollo/blob/d798f5529d2001f807e6eb1fdc37cb74e3dddc36/third_party/qt.BUILD#L16) has a similar issue to #29 so will live at a new issue (#76 )