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/automake-1.16.3/automake-1.16.3.sh

@@ -2,8 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="https://mirrors.kernel.org/gnu/automake/automake-1.16.3.tar.xz"
-
 src_prepare() {
     AUTOMAKE=automake-1.15 ACLOCAL=aclocal-1.15 AUTOCONF=autoconf-2.69 AUTOM4TE=autom4te-2.69 ./bootstrap
 

sysc/automake-1.16.3/sources

@@ -0,0 +1 @@
+https://mirrors.kernel.org/gnu/automake/automake-1.16.3.tar.xz ff2bf7656c4d1c6fdda3b8bebb21f09153a736bcba169aaf65eab25fa113bf3a