make 'require ::Foo::Bar' die

make 'require ::Foo::Bar' die

Originally, 'require ::Foo::Bar' would try to load /Foo/Bar.pm.

The previous commit changed it so that  ::Foo::Bar mapped to Foo::Bar,
and so loaded Foo/Bar.pm in the @INC path.

This commit takes the different approach of, instead of mapping, making
any bareword require that starts with '::' into an error instead.

It introduces a new error message:

    $ perl -e'require ::Foo::Bar'
    Bareword in require must not start with a double-colon: "::Foo::Bar"
    $

See the thread at:
    http://www.nntp.perl.org/group/perl.perl5.porters/2012/07/msg189909.html

( I originally used '"::"' rather than 'a double-colon', but that
made the message a bit unpenetrable:

    Bareword in require must not start with "::": "::Foo::Bar"
)