Consolidate logic to detect python interpreter (qtdeclarative, qtwebengine)

Both qtwebengine and qtdeclarative need a Python interpreter at build time. Anyhow, both use separate logic, which leads to different interpreters being detected for me:

Python3_EXECUTABLE:FILEPATH=C:/Users/kakoehne/AppData/Local/Microsoft/WindowsApps/python3.exe
QT_INTERNAL_DECLARATIVE_PYTHON:STRING=C:/Python310/python.exe

Note that python3.exe for me is only a link to launch the Windows app store, so Qt WebEngine does not compile by default.

Easiest fix would be to use the same logic in qtdeclarative also for qtwebengine:

qt_find_package(Python REQUIRED)
if(Python_Interpreter_FOUND)
    # Need to make it globally available to the project
    set(QT_INTERNAL_DECLARATIVE_PYTHON "${Python_EXECUTABLE}" CACHE STRING "")
endif()