Toolchain becomes package set part2

This is mostly a search and replace exercise (and thus probably correct). (Individual commits deal with kind-of single functions, and their associated tests.)

More complicated changes I have tried to move into separate merge requests.

mpsd-software.py

@@ -973,9 +973,9 @@ def install_environment(
             )
 
 
-def remove_environment(release, toolchains, target_dir):
+def remove_environment(release, package_sets, target_dir):
     """Remove release from installation."""
-    msg = f"Removing release {release} with toolchains {toolchains} from {target_dir}"
+    msg = f"Removing release {release} with package_sets {package_sets} from {target_dir}"
     logging.info(msg)
     raise NotImplementedError(msg)