Version Information

• vyper Version (output of vyper --version): commit 4b4e188ba83d28b5dd6ff66479e7448e5b925030

Issue description

While it is possible to import events and struct types both from .vy
and .vyi files, it is not possible to import flags.

For interfaces, this is because InterfaceT does not hold the flags
defined in the interface and hence InterfaceT.get_type_member() only
returns the set of events and struct types defined.

For modules, similarly, flag types are not added to the set of members
of the ModuleT at construction time.