Assets resolution inconsistency in Dev and Build modes

[aantipov]

Describe the bug

When a markdown file has sibling assets files

- index.md
- image.png

and assets are referenced as

![An image](image.png)

then those assets

• are resolved just fine in DEV mode
• are NOT resolved during BUILD and BUILD fails with an error message like

Referencing assets using relative paths ![An image](./image.png) fixes the problem

Reproduction

Described above

Expected behavior

Dev and Build modes should behave the same way.
If assets should be referenced using relative paths, then BOTH modes should fail at resolving absolutely referenced assets

System Info

System:
    OS: macOS 11.6
    CPU: (8) x64 Intel(R) Core(TM) i5-1038NG7 CPU @ 2.00GHz
    Memory: 126.73 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 16.13.0 - ~/.nvm/versions/node/v16.13.0/bin/node
    Yarn: 1.22.15 - ~/.nvm/versions/node/v16.13.0/bin/yarn
    npm: 8.1.0 - ~/.nvm/versions/node/v16.13.0/bin/npm
  Browsers:
    Brave Browser: 95.1.31.88
    Chrome: 96.0.4664.55
    Firefox: 90.0.2
    Safari: 15.1
  npmPackages:
    vitepress: ^0.20.1 => 0.20.1

Additional context

No response

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