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/pkg-config-0.29.2/pkg-config-0.29.2.sh

@@ -2,8 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="http://gentoo.osuosl.org/distfiles/pkg-config-0.29.2.tar.gz"
-
 src_prepare() {
     AUTOMAKE=automake-1.15 ACLOCAL=aclocal-1.15 autoreconf-2.69 -fi
 }

sysc/pkg-config-0.29.2/sources

@@ -0,0 +1 @@
+http://gentoo.osuosl.org/distfiles/pkg-config-0.29.2.tar.gz 6fc69c01688c9458a57eb9a1664c9aba372ccda420a02bf4429fe610e7e7d591