qmltyperegistrar: Warn about duplicate exports

We need to make sure we warn when something with the same name and revision is exported inside a module and ideally error out. This has happened in QtMultimedia for example and can break tooling and probably the engine as well.

We have to consider legitimately ambiguous types, though. For example, QtQuick.Dialogs insists on having multiple types of the same name, between which it discerns at run time by file selectors (+Fusion, +Basic, etc).