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.

Implement a trivial plugin registration to remove the dependency on the
`extensions` module. Also remove some unused code and options. Add
dependency on `requests` module (from LCG) to ensure the jira plugin
works correctly.

Former-commit-id: 2db86708

gen_klass of AlgTool: Remove obsolete usage of StoreGateSvc (done by AthAlgTool base now) (PyUtils-00-13-22)