email: image copied into output directory

Bug description

Given a Quarto project:

project:
  output-dir: output

with a single Markdown file:

---
title: customized
format: email
---

Not the message.

::: {.email}

::: {.subject}
The subject.
:::

The message.

![](quarto.png)

:::

and the quarto.png, when rendered, the output directory contains:

.output_metadata.json
email-preview
index.html
index_files
quarto.png

Is is not necesssary for the quarto.png to be included in the output directory.

1. The .output_metadata.json file contains the base64 encoded data for this file in its rsc_email_images field as img1.png. This is used to embed the image into an email message.
2. The email-preview contains the base64 image data within its HTML.

Note: It is not required that email-preview embeds the base64 data, but that choice was made by @rich-iannone to make the preview stand-alone.

Steps to reproduce

Create and render the described project.

Expected behavior

The quarto.png file is not included in the output directory, but has its data embedded in the .output_metadata.json and email-preview.html files.

Actual behavior

The quarto.png file is included in the output directory.

Your environment

No response

Quarto check output

Quarto 1.4.549
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.11: OK
      Dart Sass version 1.69.5: OK
      Deno version 1.37.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.4.549
      Path: /Applications/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (external install)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/aron/Library/TinyTeX/bin/universal-darwin
      Version: 2022

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

[✓] Checking Python 3 installation....OK
      Version: 3.11.7
      Path: /usr/local/opt/python@3.11/bin/python3.11
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking R installation...........OK
      Version: 4.3.1
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/library
      knitr: 1.45
      rmarkdown: 2.25

[✓] Checking Knitr engine render......OK