Change sources getting method.

- Rather than defining the urls where they are gotten (python sysa,
  python sysc, inside sysc), a spec file is now used that is easily
  interpretable and tool-independent.
- This is interpreted by rootfs.py and inside sysc.
- This is also used to make sources available and extract sources.
- Manual dirname selection is no longer required as is tarball renaming
  upon download - all of this is handled automatically.

Fixes #188

sysc/perl-5.32.1/perl-5.32.1.sh

@@ -3,9 +3,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="http://www.cpan.org/src/5.0/perl-5.32.1.tar.xz
-  http://deb.debian.org/debian/pool/main/p/perl/perl_5.32.1.orig-regen-configure.tar.gz"
-
 src_prepare() {
     default