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.

sysc/bison-2.3/bison-2.3.sh

@@ -3,7 +3,7 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-EXTRA_SRCS="gnulib-b28236b.tar.gz"
+EXTRA_DISTFILES="gnulib-b28236b.tar.gz"
 
 src_prepare() {
     default