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/libarchive-3.5.2/libarchive-3.5.2.sh

@@ -2,8 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="http://libarchive.org/downloads/libarchive-3.5.2.tar.xz"
-
 src_prepare() {
     default
 

sysc/libarchive-3.5.2/sources

@@ -0,0 +1 @@
+http://libarchive.org/downloads/libarchive-3.5.2.tar.xz f0b19ff39c3c9a5898a219497ababbadab99d8178acc980155c7e1271089b5a0