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/gettext-0.21/gettext-0.21.sh

@@ -3,9 +3,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="https://mirrors.kernel.org/gnu/gettext/gettext-0.21.tar.xz
- https://git.savannah.gnu.org/cgit/gnulib.git/snapshot/gnulib-7daa86f.tar.gz"
-
 src_prepare() {
     find . -name '*.info*' -delete
     find . -name '*.gmo' -delete

sysc/gettext-0.21/sources

@@ -0,0 +1,2 @@
+https://mirrors.kernel.org/gnu/gettext/gettext-0.21.tar.xz d20fcbb537e02dcf1383197ba05bd0734ef7bf5db06bdb241eb69b7d16b73192
+https://git.savannah.gnu.org/cgit/gnulib.git/snapshot/gnulib-7daa86f.tar.gz 2d911c2f2ed97b347d6d360b742abdc98aa626d4f8f847ee682c7cde12e90871