[perl #105922] Allow any string before ->meth

[perl #105922] Allow any string before ->meth

The rules for filtering out what do not look like package names are
not logical and disallow valid things like "::main", while allowing
q"_#@*$!@*^(".

This commit simply lets any non-empty string be used as a package
name.  If it is a typo, you’ll get an error anyway.  This allows
autobox-style calls like "3foo"->CORE::uc, or even "3foo"->uc if you
set up @ISA first.

I made an exception for the empty string because it messes up caches
somehow and causes subsequent method calls all to be called on the
main package.  I haven’t looked into that yet.  I don’t know whether
it’s worth it.

The changes to the tests in cpan/Test-Simple have been submit-
ted upstream.