This was accidentally broken by #9627 because during config sync, a config
validation happens that uses `--define System.ZonesStageVarDir=...` which fails
on the now frozen namespace.
This commit changes this to use `Internal.ZonesStageVarDir` instead. After all,
this is used for internal functionality, users should not directly interact
with this flag.
Additionally, it no longer freezes the `Internal` namespace which actually
allows using `Internal.ZonesStageVarDir` in the first place. This also fixes
`--define Internal.Debug*` which was also broken by said PR. Freezing of the
`Internal` namespace is not necessary for performance reasons as it's not
searched implicitly (for example when accessing `globals.x`) and should users
actually interact with it, they should know by that name that they are on their
own.
fd1aa73d25