I want to add a local package dependency using R devtools. The suggested way to add packages to the package DESCRIPTION file is one of the two functions use_package() and use_dev_package(), from the usethis package. The latter adds a dependency on an in-development package. The function help shows the following prototype:

use_dev_package(package, type = "Imports", remote = NULL)

where remote is

a character string to specify the remote, e.g. ‘"gitlab::jimhester/covr"’, using any syntax supported by the remotes package.

The remotes vignette shows the following

# Local
Remotes: local::/pkgs/testthat

So the command should be along these lines:

use_dev_package(foopack, type = "Imports", remote = "local::<foopack>")

However, what should be the path to the foopack. An absolute one or relative to project dir? The root package directory or the R directory with the code, or perhaps the foopack.tar.gz build?
All attempts failed for me.

Needless to say that, beyond having the local dependency properly listed in the DESCRIPTION file, I need it to be seen by the devtools build & check functions.

Edit

As regards use_dev_package(), I found a solution: if I use devtools::check(), then the dependency appears in the search path, and use_dev_package() does not complain any more (see answer below).

However, it is still unclear to me what arguments should I use to make a development check() for the main package, in particular when the package has a vignette.

Ideally, I should be able to pass the check with local dependencies by passing cran = FALSE, but this still gives "Package required but not available".

Let us assume that your main package depends on a local package `foopack".
To declare this dependency, first, install the dependency package from the local source:

install.packages("path/to/foopack", repos = NULL)

Then add the local package dependency twice to the package DESCRIPTION file, in the Imports and in the Remotes sections, like below (adjust the local:: path as needed):

Imports: 
    foopack,
Remotes: 
    local::../foopack

The DESCRIPTION file editing can also be automated with the package usethis:

usethis::use_dev_package("foopack", remote ="local::../foopack")

Now, devtools::check() gives no errors/warnings. Of course, if checking for CRAN incoming feasibility with:

devtools::check(incoming = TRUE, remote = TRUE)

one gets

Unknown, possibly misspelled, fields in DESCRIPTION:
  ‘Remotes’

Strong dependencies not in mainstream repositories:
  foopack

since Remotes is not an official description field and dependencies should be publicly available for submission on CRAN,