The module with_fallback.pm was kind of clunky and required a transfer
module.  This change replaces if with a much more generic pragma type
module, which simply appends given directories to @INC (as opposed to
the 'lib' pragma, which prepends the directories to @INC).

This also supports having a file MODULES.txt with sub-directories to
modules.  This ensures that we don't have to spray individual module
paths throughout our perl code, but can have them collected in one
place.

(do note that there is a 'fallback' module on CPAN.  However, it isn't
part of the core perl, and it has no support the any MODULES.txt kind
of construct)

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9826)