detecting external resources needs to consider css argument

[cderv]

This is because css can be processed by bslib in pre_knit and/or pre_processor, including when rendering md file in temp directory

Closes #2443
Closes #2447

The real issue is not with shiny, but rather how renders and resource discovery works when intermediates_dir is passed, and when it is another directory. This happens with shiny rendering but could happen in other situations too, not even just when intermediates is a temp directory.

Error without shiny example

• Markdown file

---
title: "Title"
output:
  html_document:
    css: styles.scss
    theme: darkly
---

### Another title

```{r}
xfun::file_string("README.md")
```

• scss file

body {background-color: purple;}

** rendering

> tmp <- tempfile(); rmarkdown::render("test.Rmd", intermediates_dir = tmp, output_dir = tmp)
Error in `sass::sass_file()`:
! Could not find file: 'styles.scss' in dir: C:/Users/chris/AppData/Local/Temp/RtmpcPUFmU
Run `rlang::last_trace()` to see where the error occurred.
> rlang::last_trace()
<error/rlang_error>
Error in `sass::sass_file()`:
! Could not find file: 'styles.scss' in dir: C:/Users/chris/AppData/Local/Temp/RtmpcPUFmU
---
Backtrace:
     ▆
  1. └─rmarkdown::render("test.Rmd", intermediates_dir = tmp, output_dir = tmp)
  2.   └─output_format$intermediates_generator(original_input, intermediates_dir) at rmarkdown/R/render.R:516:4
  3.     └─rmarkdown:::copy_render_intermediates(...) at rmarkdown/R/html_document_base.R:159:4
  4.       └─rmarkdown::find_external_resources(original_input) at rmarkdown/R/html_resources.R:404:2
  5.         └─rmarkdown:::discover_rmd_resources(input_file, discover_single_resource) at rmarkdown/R/html_resources.R:108:4
  6.           └─rmarkdown::render(...) at rmarkdown/R/html_resources.R:339:2
  7.             └─output_format$pre_processor(...) at rmarkdown/R/render.R:845:6
  8.               └─rmarkdown (local) base(...) at rmarkdown/R/output_format.R:126:6
  9.                 ├─sass::sass(...) at rmarkdown/R/html_document_base.R:103:8
 10.                 │ └─sass::as_sass(input)
 11.                 │   ├─htmltools::attachDependencies(...)
 12.                 │   │ └─htmltools:::asDependencies(value)
 13.                 │   └─sass:::find_dependencies(input)
 14.                 │     └─sass:::is_sass_bundle_like(x)
 15.                 │       └─sass::is_sass_bundle(x)
 16.                 └─sass::sass_file(f)

It would happen with tmp <- "dummy" too.

This happens in our finding resources processing, because we render to an .md file in a temp directory, and we need to move the resource in this temp folder at this time. css or scss could be processed by bslib or sass in pre_knit and/or pre_processor, including when rendering md file in temp directory. We need to account for this.

@cpsievert this PR closes #2448 without merging as this is a broader fix, even when shiny is not used.

[cderv]

Checks triggered manually : https://github.com/rstudio/rmarkdown/actions/runs/5005228987