QML DOM: overhaul builtin handling

The handling of builitins is out of sync between the compilers and the DOM since https://codereview.qt-project.org/c/qt/qtdeclarative/+/622385 .

"builtins" and "QML" are actually separate, and only what has been termed "hardcoded" buildins should always be available.

This also leads to the issue that the DOM will attempt to import both builtins.qmltypes (becasue that's hardcoded in DomEnvironment::loadBuiltins ) and QML/plugins.qmltypes (which is a dependency of QtQml).

That leads in turn to errors, because QtObject is defined multiple times.