mdist: use better approach to finding original configured options
mdist: use better approach to finding original configured options

Instead of reading intro-buildoptions.json, a giant json file containing
every option ever + its current value, use the private file that is
internally used by msetup for e.g. --wipe to restore settings.

This accurately tracks exactly the options specified on the command
line, and avoids lengthy summary messages containing all the overridden
defaults.

It also avoids passing potentially incompatible options, such as
explictly specifying -Dpython.install_env while also having a non-empty
-Dpython.{x}libdir

Fixes #10181

mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson