Keep copyDependencyToDir() from creating empty directories

[gadenbuie]

When an htmlDependency() doesn't have any files, copyDependencyToDir() will still create an empty directory, as in the reprex below. This PR fixes this behavior by cleaning up the target directory when the dependency doesn't have any source files.

We still temporarily create the target directory and update the dependencies $src$file value, though, because rmarkdown will eventually call makeDependencyRelative() and we need $src$file to be a child of the document's basedir or an error is thrown.

---
output: 
  html_document:
    self_contained: no
---

```{r echo=FALSE}
# create empty temp dir
tmpdir <- tempfile()
dir.create(tmpdir)

# add an htmldependency from that tempdir
htmltools::tagList(
  htmltools::htmlDependency(
    name = "empty",
    version = "9.9.9",
    src = tmpdir,
    head = "<script>alert('boo')</script>",
    all_files = FALSE
  )
)
```

[MoLi7]

I suppose this PR should fix the mysterious empty directories created after knitting a xaringan and xaringanExtra powered presentation, as described in this StackOverflow question.

Could someone merge this PR to implement @gadenbuie's proposal? Thank you!

[SNAnalyst]

I don't know it this adds any additional information, but I have found that this problem occurs when I use

xaringanextra::use_clipboard()

(regardless of whether I add htmltools::tagList() after this. So, just xaringanextra::use_clipboard() creates this behavior by itself already.

[gadenbuie]

@cpsievert I made the change requested by @schloerke, this should be good to go now!

[cpsievert]

@gadenbuie thanks! Please also add a NEWS item and then I'll merge

[cpsievert]

@schloerke any idea what's going wrong here? https://github.com/rstudio/htmltools/runs/4193559908?check_suite_focus=true#step:5:1270

[schloerke]

@cpsievert I have no clue. 😞

It is extremely odd. It is tied to r-lib/actions/setup-r-dependencies in the line: pak::pkg_deps("local::.", dependencies = TRUE) when making the dependencies file for caching.

Why it is odd is that every other linux job works. I also busted the cache by incrementing the cache number, still no improvement. Also removed `extra-packages to be installed afterwards, still no improvement.

[cpsievert]

@gadenbuie would you mind rebasing/merging with main so this gets put in the right place?

[gadenbuie]

@cpsievert ✅