[perl #88132] broken ISA lookup after aliasing packages ending with ::

[perl #88132] broken ISA lookup after aliasing packages ending with ::

gv_fetchpvn_flags did not always assign a name to a return HV ending
with ::. This would result in code in various places skipping certain
‘stashes’ (in quotes because nameless HVs are technically not stashes)
because they were nameless when they should not have been.

So sometimes ISA caches would end up being out of date, as in the test
cases posted with [perl #88132] (and incorporated into this patch).

This commit fixes that by changing the parsing of glob names.

Formerly, a :: was not considered a package separator if it came imme-
diately after a ::. So foo:::: would become foo::/:: (with the final
:: considered a regular stash entry, not a ‘stash’ stash entry) and
foo:::::: would become foo::/:::/:.

Now a :: is always a package separator. So *foo::::bar is accessible
via $foo::{"::"}{bar} and *$foo:::::: via $foo::{"::"}{"::"}.

This happens to fix [perl #88134] as well.