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/run.sh

@@ -14,6 +14,7 @@ set -e
 export PREFIX="${prefix}"
 # shellcheck disable=SC2154
 export SOURCES="${sysa}"
+export DISTFILES="${sysa}/distfiles"
 export DESTDIR=/tmp/destdir
 export REPO="${PREFIX}/src/repo"