Variety of improvements

- Rename sources to distfiles for clarity.
- Per sys(a/c) distfiles to reduce rootfs.py processing and reduce RAM
  usage in sysa.
- Canonicalise early kaem mes/tcc files to kaem script conventions.
- Cleanup unused setup in python.

sysa/musl-1.2.2/musl-1.2.2.sh

@@ -1,7 +1,9 @@
-# SPDX-FileCopyrightText: 2021 Andrius Štikonas <andrius@stikonas.eu>
+# SPDX-FileCopyrightText: 2022 Andrius Štikonas <andrius@stikonas.eu>
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
+SRCS="musl-1.2.2.tar.gz"
+
 src_configure() {
     CC=gcc ./configure \
         --host=i386-unknown-linux-musl \