[Perl/perl5] bcd084: class :isa(): check for not only a stash, but a class

[Tony Cook via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: bcd084d7d82813d8058dd52097a42fb8890939b0
      
https://github.com/Perl/perl5/commit/bcd084d7d82813d8058dd52097a42fb8890939b0
  Author: Tony Cook <t...@develop-help.com>
  Date:   2023-08-14 (Mon, 14 Aug 2023)

  Changed paths:
    M MANIFEST
    M class.c
    M t/class/inherit.t
    A t/lib/class/A.pm
    A t/lib/class/A/B.pm

  Log Message:
  -----------
  class :isa(): check for not only a stash, but a class

If class Foo::Bar exists then the Foo stash exists, so checking
for that when Foo is the requested base class is meaningless.

Instead try to load the base class if it is not known as a class, not
just if it's not a stash, just as we do immediately following.

Fixes #21332