(PUP-1220) make the dynamic scoping fix for #22800 flexible

As suggested by Henrik Lindberg, make the parser only behave correctly
when parser=future is configured. If working manifests rely on the
regression, the next point release won't break them. Instead, users can
test for this using the configuration setting.

The next major release should adopt the fix as standard behavior and
the legacy support should be removed. Since the variable lookups in
templates relies on the accessing of local attributes, there is no
way to issue deprecation warnings to users.

The spec test for the fix is now pending.