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/gc-8.0.4/gc-8.0.4.sh

@@ -2,8 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="https://www.hboehm.info/gc/gc_source/gc-8.0.4.tar.gz"
-
 src_prepare() {
     autoreconf-2.71 -fi
 }

sysc/gc-8.0.4/sources

@@ -0,0 +1 @@
+https://www.hboehm.info/gc/gc_source/gc-8.0.4.tar.gz 436a0ddc67b1ac0b0405b61a9675bca9e075c8156f4debd1d06f3a56c7cd289d