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/libtool-2.4.7/libtool-2.4.7.sh

@@ -4,8 +4,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="http://mirrors.kernel.org/gnu/libtool/libtool-2.4.7.tar.xz
- http://git.savannah.gnu.org/cgit/gnulib.git/snapshot/gnulib-a521820.tar.gz"
 
 src_prepare() {
     default

sysc/libtool-2.4.7/sources

@@ -0,0 +1,2 @@
+http://mirrors.kernel.org/gnu/libtool/libtool-2.4.7.tar.xz 4f7f217f057ce655ff22559ad221a0fd8ef84ad1fc5fcb6990cecc333aa1635d
+http://git.savannah.gnu.org/cgit/gnulib.git/snapshot/gnulib-a521820.tar.gz 719b399fe09a8f6ca14ba8c4a9a60ce9f93f4892effb50961ef3d8cd1a33ff65