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/tar-1.34/sources

@@ -0,0 +1,2 @@
+http://mirrors.kernel.org/gnu/tar/tar-1.34.tar.xz 63bebd26879c5e1eea4352f0d03c991f966aeb3ddeb3c7445c902568d5411d28
+http://git.savannah.gnu.org/cgit/gnulib.git/snapshot/gnulib-30820c.tar.gz df807e694deea2dcba0c43af318394f3e3fcd52658c3b71b61dad0ce0c0cfb77

sysc/tar-1.34/tar-1.34.sh

@@ -3,9 +3,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="http://mirrors.kernel.org/gnu/tar/tar-1.34.tar.xz
- http://git.savannah.gnu.org/cgit/gnulib.git/snapshot/gnulib-30820c.tar.gz"
-
 src_prepare() {
     default