Qt Quick Controls 2 was named that way because it was a follow-up to
Qt Quick Controls 1.x. Now that Qt Quick Controls 1 is no longer
supported, we don't need to have "2" in the name. Work on this was
already started for the documentation in
1abdfe5d5a052f2298b7bf657513dfa7e0c66a56.

By doing this renaming a few weeks before feature freeze, it won't
affect the release but still results in as little time possible spent
manually fixing conflicts in cherry-picks from non-LTS releases as a
result of the renaming.

This patch does the following:

- Renames directories.
- Adapts CMakeLists.txt and other files to account for the new paths.

A follow-up patch will handle documentation.

It does not touch library names or other user-facing stuff, as that
will have to be done in Qt 7.

Task-number: QTBUG-95413
Change-Id: I170d8db19033ee71e495ff0c5c1a517a41ed7634
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

Replace the current license disclaimer in files by
a SPDX-License-Identifier.
Files that have to be modified by hand are modified.
License files are organized under LICENSES directory.

Pick-to: 6.4
Task-number: QTBUG-67283
Change-Id: I63563bbeb6f60f89d2c99660400dca7fab78a294
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

Remove usages of outdated LGPL3 header that references LICENSES.LGPLv3
instead of LICENSES.LGPL3.

Change-Id: If7a90ec18331a68491c5a740f131a5e7b3f37df4
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

The new qml CMake API places a closer relationship between
the backing target and the plugin target. Both are typically
created together and they share a lot of common details.
Instead of creating them in different parts of the source
tree, they are now specified together. The src/imports
area has effectively been absorbed into the other
corresponding subdirectories below src with this change.

Task-number: QTBUG-91621
Change-Id: I9bd32e9eb78c198ccc9db04e2829303cac323502
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>