As per a recent email to the Qt Development lists, projects/modules must now fix qml header and symbol naming to match the new structure. Currently there is a deprecation mechanism in place but it will soon be removed.
QTBUG-23737 for more details.
The renaming changes have now been merged from the api_changes branch of qtdeclarative into the master branch.
At this time, the declarative module is pinned to the api_changes branch of qtbase, but this will be removed when the merge of qtbase/api_changes to qtbase/master gets through CI.
We expect that projects using the existing QDeclarative... names for these classes should continue to build correctly in the immediate future, but the backwards compatibility workarounds created to enable this will be removed in the near future. Please update you projects to use the new naming scheme; the shell script in qtdeclarative/bin/rename-qtdeclarative-symbols.sh should make the correct changes in client code.