acmd.py loads all available plugins (modules in its script directory)
each time it is executed. Removing unnecessary imports at the module
level speeds up this process significantly. For example, removing the
unnecessary ROOT import in diff_root_files.py saves 2 out of the 3
seconds when running `acmd.py -h`.


Former-commit-id: 788c04c2663a88b460897512e6d4c42bd5b5b246