Error in relativeTo(basepath, dir) rmarkdown libs

I want to have one libs dir at root, and then my slides saved in subdirectories, but I get this error

```
Error in relativeTo(basepath, dir) :
The path /Users/luna/slides/libs/remark-css does not appear to be a descendant of /Users/luna/slides/papers/
Calls: ... html_dependencies_as_string -> lapply -> FUN -> relativeTo
Execution halted
```
See issue https://github.com/rstudio/rmarkdown/issues/146
And this:
https://stackoverflow.com/questions/24610740/rmarkdown-different-output-folders-shared-libs

Start by reproducing the relativeTo(basepath, dir) error from the Rmd slides and review the linked rmarkdown issue and Stack Overflow discussion. Done means slides in subdirectories can use a root-level libs directory without the descendant-path error.

Written by the indexing model from the issue text.