The plugins should declare that they handle a command through this helper.

@param [String] command being handled by them @param [Class] (optional) class that handles the command. If not

                provided, the `self` class will be used.

The plugins should declare that they provide a installation source through this helper.

@param [String] the source type they provide @param [Class] (optional) class that handles the source. If not

                provided, the `self` class will be used.

The cache dir to be used by the plugins for storage

@return [Pathname] path of the cache dir

A tmp dir to be used by plugins Accepts names that get concatenated as suffix

@return [Pathname] object for the new directory created