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/linux-headers-5.10.41/linux-headers-5.10.41.sh

@@ -4,7 +4,7 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 
 src_unpack() {
-    tarball="/sources/linux-headers-5.10.41.tar.gz"
+    tarball="${DISTFILES}/linux-headers-5.10.41.tar.gz"
     tar -xzf "${tarball}" "${dirname}/scripts"
     tar -xzf "${tarball}" "${dirname}/include"
     tar -xzf "${tarball}" "${dirname}/arch/x86/include"