qt: Add Qt:: namespace alias for every module.

[cadmiy]

Summary

Changes to recipe: qt/5.x.x, qt/6.x.x

Motivation

Qt6 supports Qt:: namespace for cmake targets. This namespace has also been backported to Qt5.
However, the namespace is not available in conan recipes. The problem arose when we were trying to switch from Qt5 to Qt6.

Details

Inside Qt5{module}Config.cmake files for every module there is an alias for Qt5/Qt6 namespace to Qt. For example, for Qt5CoreConfig.cmake it looks like this:

# Add a versionless target, for compatibility with Qt6.
if(NOT "${QT_NO_CREATE_VERSIONLESS_TARGETS}" AND TARGET Qt5::Core AND NOT TARGET Qt::Core)
    add_library(Qt::Core INTERFACE IMPORTED)
    set_target_properties(Qt::Core PROPERTIES
        INTERFACE_LINK_LIBRARIES "Qt5::Core"
    )
endif()

However, this alias is not available in conan recipes. The suggested changes fix that.

---

• Read the contributing guidelines
• Checked that this PR is not a duplicate: list of PRs by recipe
• Tested locally with at least one configuration using a recent version of Conan

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