Prework

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Description

After #481, most targets functions have arguments to specify the target script path and data store path. These arguments work by temporarily changing global config settings for the function call. In an ideal world, the YAML config settings should just set default arguments. The YAML should serve the arguments, not the other way around.

Also, the config argument of tar_config_set() sets a transient value that vanishes when the session restarts, which breaks the session independence that targets is supposed to follow.

I intend to do a bunch of cleanup for all this. It should simplify some of the infrastructure, e.g. getting rid of the config class.