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/curl-7.83.0/curl-7.83.0.sh

@@ -2,8 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="http://master.dl.sourceforge.net/project/curl.mirror/curl-7_83_0/curl-7.83.0.tar.xz"
-
 src_prepare() {
     default
 

sysc/curl-7.83.0/sources

@@ -0,0 +1 @@
+http://master.dl.sourceforge.net/project/curl.mirror/curl-7_83_0/curl-7.83.0.tar.xz bbff0e6b5047e773f3c3b084d80546cc1be4e354c09e419c2d0ef6116253511a