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

@@ -1134,7 +1134,7 @@ def main():
             if cmd in ["install", "reinstall", "remove"]:
                 # "install" command needs additional documentation
                 package_set_help = (
-                    f"One or more toolchains to command {cmd}. "
+                    f"One or more package sets (like toolchains) to be {cmd}ed. "
                     "Use 'ALL' to refer to all available package sets."
                 )