perl: allow empty lines and comments in perl-rdepends.txt

With this change the rdepends file can now have empty lines
and comment lines. The perl-rdepends.txt generation will be
fixed with further commits to leverage this change.

Signed-off-by: Awais Belal <awais_belal@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>