Derive C++ namespaces from each IDL module's location and use those
qualified names when generating binding code.
Also Teach dictionaries their owning IDL module path so dictionary C++
types can be qualified the same way as interfaces. This removes the need
for the generated `using namespace Web::*` hack and the hard-coded
namespace list.
Also fix DOMURL.idl to refer to the IDL interface name `URL`, not the
C++ implementation name `DOMURL`.
535b8f5b9b