Fix the behaviour of require WRT to global symbols

[jonathanstowe]

Recent changes in require meant that code that did a require in an inner lexical scope were not having the symbols merged in such a way that they could be found by an indirect lookup, this broke for instance, the way that Panda dealt with the Build.pm files

[ugexe]

This PR allows IO::Socket::SSL to be installed again, which reveals more incorrect behavior:

nickl@li685-90:~/perl6$ perl6 -v
This is Rakudo version 2015.12-323-gbb2953c built on MoarVM version 2016.01-28-g2136293
implementing Perl 6.c.

nickl@li685-90:~/perl6$ perl6 -e 'require IO::Socket::SSL; say 42; say (try require IO::Socket::SSL);'
42
(SSL)

nickl@li685-90:~/perl6$ perl6 -v
This is Rakudo version 2015.12-380-g6159b5e built on MoarVM version 2016.01-28-g2136293 implementing Perl 6.c.

nickl@li685-90:~/perl6$ perl6 -e 'require IO::Socket::SSL; say 42; say (try require IO::Socket::SSL);'
42
Nil