devel/cmake*: Pet portclippy(1) and migrate to Qt6

Migrate towards Qt6:

• Switch the default flavor of devel/cmake-gui to qt6.
• Use qt6-tools instead of qt5-help in devel/cmake-doc to generate the Qt help file.

Depend on the flavorized version of textproc/py-sphinx.

Pet portclippy(1) and portlint(1) and make Makefile order more consistent
within the devel/cmake* ports. Remove duplicate DISTINFO_FILE line from
devel/cmake-man. [1]

Make build of devel/cmake-core verbose [2] and respect WITH_CCACHE_BUILD
in bootstrapping phase.

Remove the "experimental" annotation from the CPACK option in
devel/cmake-core to avoid end-user confusion. [3] The FreeBSD CPack
generator has been marked experimental since version 3.11.4, but is
supported upstream and shouldn't be considered experimental even if
we've had to make some local changes to it. If it isn't working
correctly, please file a bug report.

PR: 276792 [1], 276782[2], 274640 [3]
Reported by: diizzy [1], Anton Saietskii <vsasjason@gmail.com> [2],

		Martin Waschbüsch <martin@waschbuesch.de> [3]