modelbuilder: correctly detect property name conflicts

The tricky part is the handling of existing constructors.

Signed-off-by: Jean Privat <jean@pryen.org>
tests: update base_virtual_type_redef.res

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add error_prop_name*

Signed-off-by: Jean Privat <jean@pryen.org>
Merge: Correct detection of property name conflicts

Fixes #395
Pull-Request: #398