In order to unify the handling and reuse the CLITool infrastructure
the plugin manager should make use of this as well.
This obsolets the -i and --install options but requires the user
to use `install` as the first argument of the CLI.
This is basically just a port of the existing functionality, which
is also the reason why this is not a refactoring of the plugin manager,
which will come in a separate commit.
2f54b89a23