Callr module sourcing

I've been having trouble working with callr and box. Specifically, sourcing modules by `box::use(./module)' fails, as it appears that callr attempts to find them in a /tmp/ folder, rather than the working directory.

Might be related to #49?

Background info: I've come across this when using the {targets} package, where I have to specify `callr_function = NULL` in order to run the pipeline

Start by reproducing the failure with callr and `box::use(./module)`, then compare how the working directory and `/tmp/` are handled. Check the relationship to #49 and the `targets` case; done means relative module sourcing works under callr without requiring `callr_function = NULL`.

Written by the indexing model from the issue text.