Avoid loading modules for %! and %+ on meeting %{"foo::!"} and %{"foo::+"}

Avoid loading modules for %! and %+ on meeting  %{"foo::!"} and %{"foo::+"}

Previously, just %{"foo::!"} would not trigger a load of Errno, but
${"foo::!"}; %{"foo::!"}; would, due to the different code paths taken through
Perl_gv_fetchpvn_flags(). As the modules themselves are responsible for calling
tie on the relevant global variables, there never was a problem with the wrong
variables *getting* their behaviour. However, the attempted load of the XS
module Tie::Hash::NamedCapture for %{"foo::-} meant that t/op/leaky-magic.t
would not pass under minitest. This commit resolves that failure.