PyUtils: Improve startup time of acmd.py and delete unused modules

In most cases it is not necessary to load all plugins as the user very likely specified one on the command line. Try to first load a specific plugin and then fall back to loading all of them. This reduces the startup time significantly, i.e. if we keep adding new sub-commands.

Also remove the "running sub-command" INFO message as this makes it impossible to implement sub-commands that can be used in pipes.

Delete unused third-party modules: pshell, reimport, smem