use_standalone() is useful outside packages

[daranzolin]

I've started running usethis::use_standalone("rlib/rlang", file = "types-check") within my {targets} projects. It works great because {targets} sources the files in R/ each run. Thus, use_standalone() prints a superfluous error message:

> use_standalone("r-lib/rlang", file = "types-check")
✔ Setting active project to "<redacted>".
✔ Writing R/import-standalone-types-check.R.
✔ Writing R/import-standalone-obj-type.R.
Error: No root directory found in <redacted> or its parent directories. Root criterion: contains a file "DESCRIPTION" with contents matching "^Package: "

[jennybc]

use_standlone() only explicitly checks for project-hood, but, yes, it later got some logic for adding the dependencies of the standalone file and that implicitly checks for a package.

So we should skip those actions in the absence of a DESCRIPTION file (and probably emit some messaging?).

(In the meantime, it's also perfectly valid to have a DESCRIPTION file in a non-package project, so you could experiment with placing one to give use_standalone() something to act on.)