wasm: declarative app builds result in "Qt6::qt_plugins-NOTFOUND".

When building the quick animation example for webassembly, using the WIP patch for cmake support for wasm, I get this error:

CMake Error at /Users/lpotter/development/builds/qt5-base/lib/cmake/Qt6Qml/Qt6QmlPlugins.cmake:48 (get_target_property):
get_target_property() called with non-existent target
"Qt6::qt_plugins-NOTFOUND".
Call Stack (most recent call first):
/Users/lpotter/development/builds/qt5-base/lib/cmake/Qt6Qml/Qt6QmlConfig.cmake:122 (include)
/Applications/CMake.app/Contents/share/cmake-3.19/Modules/CMakeFindDependencyMacro.cmake:47 (find_package)
/Users/lpotter/development/builds/qt5-base/lib/cmake/Qt6Quick/Qt6QuickDependencies.cmake:86 (find_dependency)
/Users/lpotter/development/builds/qt5-base/lib/cmake/Qt6Quick/Qt6QuickConfig.cmake:40 (include)
/Users/lpotter/development/builds/qt5-base/lib/cmake/Qt6/Qt6Config.cmake:80 (find_package)
CMakeLists.txt:20 (find_package)

CMake Error at /Users/lpotter/development/builds/qt5-base/lib/cmake/Qt6Qml/Qt6QmlPlugins.cmake:54 (get_target_property):
get_target_property() called with non-existent target
"Qt6::qt_plugins-NOTFOUND".
Call Stack (most recent call first):
/Users/lpotter/development/builds/qt5-base/lib/cmake/Qt6Qml/Qt6QmlConfig.cmake:122 (include)
/Applications/CMake.app/Contents/share/cmake-3.19/Modules/CMakeFindDependencyMacro.cmake:47 (find_package)
/Users/lpotter/development/builds/qt5-base/lib/cmake/Qt6Quick/Qt6QuickDependencies.cmake:86 (find_dependency)
/Users/lpotter/development/builds/qt5-base/lib/cmake/Qt6Quick/Qt6QuickConfig.cmake:40 (include)
/Users/lpotter/development/builds/qt5-base/lib/cmake/Qt6/Qt6Config.cmake:80 (find_package)
CMakeLists.txt:20 (find_package)

– Configuring incomplete, errors occurred!
See also "/Users/lpotter/development/builds/animation/CMakeFiles/CMakeOutput.log".

Digging into this, I find, on line 43 of Qt6QmlPlugins.cmake:

1. The code in here uses the properties defined in qt_import_plugins (Qt6CoreMacros.cmake)
    
   Digging into Qt6CoreMacros.cmake, on line 555 of Qt6CoreMacros:
2. TODO : support qml plug-ins.
   function(qt6_import_plugins target)