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/xbps-0.59.1/sources

@@ -0,0 +1 @@
+https://github.com/void-linux/xbps/archive/refs/tags/0.59.1.tar.gz 0cbd8d5f23a62047c75974bca21da9f004a94efffd7f37c68562a8dbc869fb2a

sysc/xbps-0.59.1/xbps-0.59.1.sh

@@ -3,7 +3,6 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 
 # TODO: add mechanism to change output filename to something nicer
-urls="https://github.com/void-linux/xbps/archive/refs/tags/0.59.1.tar.gz"
 
 src_configure() {
     PKG_CONFIG_PATH="${PREFIX}/lib/musl/pkgconfig" \