`dot_to_package()` requires a `man/` and a `R/` folder

[ha0ye]

I have some R packages that are code-less, using the structure for other reasons - basically packaging contents to use automated machinery, such as {{pkgdown}}.

When I try to use write_codemeta() to generate a codemeta.json file, it fails when an error in dot_to_package(). After checking the code, it seems that this function wants a folder that has DESCRIPTION, NAMESPACE, man/ and R/. (The last two don't exist in my repos) https://github.com/ropensci/codemetar/blob/20376ab4f017f6561544fa6ee4a603cda753e434/R/utils.R#L147

I think the last two checks can be removed.

example repo: https://github.com/uf-repro/rmarkdown-intro

[maelle]

@cboettig if you're ok with this change I can make a PR