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/sed-4.8/sed-4.8.sh

@@ -2,9 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="https://mirrors.kernel.org/gnu/sed/sed-4.8.tar.xz
- http://git.savannah.gnu.org/cgit/gnulib.git/snapshot/gnulib-d279bc.tar.gz"
-
 src_prepare() {
     rm configure
     find . -name 'Makefile.in' -delete

sysc/sed-4.8/sources

@@ -0,0 +1,2 @@
+https://mirrors.kernel.org/gnu/sed/sed-4.8.tar.xz f79b0cfea71b37a8eeec8490db6c5f7ae7719c35587f21edb0617f370eeff633
+http://git.savannah.gnu.org/cgit/gnulib.git/snapshot/gnulib-d279bc.tar.gz 12cfa21abf618a274017d6b18e95fc6582519d7c08e2403e5c5772ccdd5b85f4