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/patch-2.7.6/patch-2.7.6.sh

@@ -2,9 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="https://mirrors.kernel.org/gnu/patch/patch-2.7.6.tar.xz
- https://git.savannah.gnu.org/cgit/gnulib.git/snapshot/gnulib-e017871.tar.gz"
-
 src_prepare() {
     ../../import-gnulib.sh
 

sysc/patch-2.7.6/sources

@@ -0,0 +1,2 @@
+https://mirrors.kernel.org/gnu/patch/patch-2.7.6.tar.xz ac610bda97abe0d9f6b7c963255a11dcb196c25e337c61f94e4778d632f1d8fd
+https://git.savannah.gnu.org/cgit/gnulib.git/snapshot/gnulib-e017871.tar.gz a285dc300c3d9c25cc06e38827ef40f6073ec3b9b0fcb5bba433f943be92d8d4