### Bug description

Following up on the bug fix from #3414, the patched version of the code seems to not discover/use extension scss at all.

Given just a `_variables.scss` file that includes a single line, 

```scss
@import "definitely/not/a/file"
```

Quarto will render the document without issue, even though this file clearly should throw an error.

Possibly related - In the process of testing this out, I also couldn't seem to clear quarto's cache entirely. A previous iteration of this test used `$primary: #F00;` as the contents of the `_variables.scss` file and this is still affecting the webpage's style, even in a private browser or separate browser entirely. I've tried deleting every quarto-generated file I can find and running with `--no-cache`, but can't seem to get rid of this cached stylesheet. Perhaps this is related to the current file not throwing errors. Is there a better way to get quarto to render a project from scratch?

### Steps to reproduce

1. Clone this repo: https://github.com/dgkf/quarto-revealjs-theme-reprex (@quarto-cli-issue-5794)
2. Run `quarto preview`
3. Quarto should successfully build the webpage
4. Open `_extensions/custom/scss/_variables.scss` and observe that it should throw a SCSS file-not-found error

### Expected behavior

`_variables.scss` from the extension is picked up by quarto, resulting in an error

### Actual behavior

Site builds, and does not appear to use the theme scss

### Your environment

- MacOS 
- `quarto-cli` [b5c6ddc37cb2c34e7d6da01cff13b640e97c22cc](https://github.com/quarto-dev/quarto-cli/commit/b5c6ddc37cb2c34e7d6da01cff13b640e97c22cc) (June 2, 2023)

### Quarto check output

```
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.2: OK
      Dart Sass version 1.55.0: OK
      Deno version 1.33.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 99.9.9
      Path: /Users/kelkhofd/Projects/quarto-cli/package/dist/bin

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.11.3
      Path: /opt/homebrew/opt/python@3.11/bin/python3.11
      Jupyter: 5.3.0
      Kernels: julia-1.8

      NOTE: No Jupyter k
```

Note, I omitted errors finding R because I run R in a docker container and quarto doesn't like that it can't find it's system temp session directory. I'm pretty sure it's unrelated to this bug.
dragonstyle
2023-06-02T17:01:53Z
dgkf
bug
v1.4
closed
Extension scss isn't being discovered
https://github.com/quarto-dev/quarto-cli/issues/5794