Improve the upgrade path for Qt with portupgrade.

Newer Qt releases depend on the build tools and qmake from that release,
but the existing way of specifying that (e.g. USE_QT=buildtools_build)
could pick up the *old* Qt version's tools and qmake. Never a problem
in poudriere, which builds cleanly, but it keeps tripping up other
tools. Replace the existing definitions for the dependencies with
versioned dependencies, so that the right version of tools is built
and used for the rest of the Qt stack.

PR: 245862
Reported by: Piotr Smyrak
Reviewed by: tcberner