use file_coverage for package with internal functions

[Naikless]

I am starting to introduce unittests to a package that didn't have them before, meaning that I have only implemented them for a few of the package scripts yet.

For this reason, I would like to test coverage only for specific files of the package. As far as I understand, this should be possible with

file_coverage( source_files = "source.R",  test_files = "test-source.R")

However, I run into the problem that calls to package functions inside these tests produce could not find function errors.
Is this to be expected?

package_coverage works but takes a lot longer. Is there any other way this could be achieved?