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/bash-5.1/bash-5.1.sh

@@ -5,8 +5,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="http://mirrors.kernel.org/gnu/bash/bash-5.1.tar.gz"
-
 src_prepare() {
     # Remove bison generated files
     rm y.tab.c y.tab.h

sysc/bash-5.1/sources

@@ -0,0 +1 @@
+http://mirrors.kernel.org/gnu/bash/bash-5.1.tar.gz cc012bc860406dcf42f64431bcd3d2fa7560c02915a601aba9cd597a39329baa