Refactor file writing - arrange for use of bundled Perl modules as fallback

For our own convenience, we need a mechanism to be able to fall back
on bundled Perl modules.  It's a minimal package that's called like
this:

    use with_fallback qw(Module1 Module2 ...);

For each module, it will try to require them from the system
installation, and failing that, it will temporarly add external/perl
and try to require transfer::{ModuleName}.  It requires that each
bundled Perl modules is accompanied by a small transfer module
(external/perl/transfer/ModuleName.pm in our example) that knows
exactly what to load.

Reviewed-by: Rich Salz <rsalz@openssl.org>