Stop relying on Package::Stash PP bugs

Stop relying on Package::Stash PP bugs

We want to know if $MyApp::_config contains something like a valid
configuration, not if the package symbol exists.

The pure-perl version Package::Stash has always been buggy in assuming that a
scalar symbol containing undef is a symbol that doesn't exist. This is to work
around the fact that getting the SCALAR slot from a glob in pure perl
autovivifies the scalar to undef. Package::Stash::XS doesn't have that
limitation and is used transparently by Package::Stash if it's installed. That
exposed this bug in our code.