overriding usethis behavior

[jacobbien]

Currently, litr redefines usethis's challenge_nested_project() function as follows:

utils::assignInNamespace("challenge_nested_project", function(...) NULL, ns = "usethis")

However, in version 2.2.0 of usethis, they added functionality to allow one to have a package nested inside a project. They now have an undocumented environment variable to do this. It would be nicer to use this new approach rather than using the utils::assignInNamespace() approach.

To read more about the new approach, see here:

• Rationalize nested projects r-lib/usethis#1647
• https://github.com/r-lib/usethis/pull/1647/files