Optional feature: cleanly track functions/objects from multiple external packages

[wlandau]

Prework

• I understand and agree to the code of conduct and the contributing guidelines.
• I have already submitted an issue to the issue tracker to discuss my idea with the maintainer.
• This pull request is not a draft.

Related GitHub issues and pull requests

• Ref: A clean way to track functions from external packages #239

Summary

By default, targets ignores changes to functions inside external packages. However, if a workflow centers on a custom package with methodology under development, users can make targets automatically watch the package's functions for changes. Simply supply the names of the relevant packages to the imports argument of tar_option_set(). Unlike drake, targets can track multiple packages this way, and the internal mechanism is much safer.

If you supply multiple packages, e.g. tar_option_set(imports = c("p1", "p2")), then the objects in p1 override the objects in p2 if there are name conflicts. Similarly, objects in tar_option_get("envir") override everything in tar_option_get("imports").