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/zlib-1.2.12/sources

@@ -0,0 +1 @@
+https://zlib.net/zlib-1.2.12.tar.xz 7db46b8d7726232a621befaab4a1c870f00a90805511c0e0090441dac57def18

sysc/zlib-1.2.12/zlib-1.2.12.sh

@@ -2,8 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="https://zlib.net/zlib-1.2.12.tar.xz"
-
 src_configure() {
     ./configure --prefix="${PREFIX}" --libdir="${PREFIX}/lib/musl" --static
 }