A clean way to track functions from external packages

[wlandau]

Prework

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Motivation

#132, #235, ropensci/drake#1286. Tracking functions from packages has been an issue for years, and it's tricky because packages have their own environments. Until now, the choices have been:

1. Ignore package functions.
2. Track only the functions inside a single package, ignoring the global environment.
3. Manually assign package functions into the global environment.

(1) is severely limiting, (2) is still limiting, and (3) is a brittle hack that breaks environments and closures.

Proposal

Distinguish between the environment that runs targets and the environment to inspect for global dependencies. targets could manually create the latter while leaving the former completely untouched. By default, they could be the same. But if the user assigns tar_option_set(imports = c("analysisPackage1", "analysisPackage2")), targets could create a new environment, copy in everything from getNamespace("analysisPackage2"), do the same for getNamespace("analysisPackage1"), and then again for tar_option_get("envir"). Simple to implement and best of all worlds!