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/musl-1.2.3/musl-1.2.3.sh

@@ -3,8 +3,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="https://musl.libc.org/releases/musl-1.2.3.tar.gz"
-
 src_configure() {
     ./configure \
         --host=i386-unknown-linux-musl \

sysc/musl-1.2.3/sources

@@ -0,0 +1 @@
+https://musl.libc.org/releases/musl-1.2.3.tar.gz 7d5b0b6062521e4627e099e4c9dc8248d32a30285e959b7eecaa780cf8cfd4a4