broken `.DirIcon` is generated if `.desktop` file references icon not in the appdir root

### to reproduce
1. create an `AppDir` the following`.desktop` file:
`app.desktop`:
```ini
[Desktop Entry]
Name=test
Exec=AppRun
Icon=/usr/bin/icon
Type=Application
Categories=Development;IDE;
Terminal=false
StartupNotify=true
```
2. put an icon file at `AppDir/usr/bin/icon.svg`
3. run `ARCH=x86_64 appimagetool .`
4. attempt to open the `.DirIcon` symlink

### expected result

either:

- `.DirIcon` links to `AppDir/usr/bin/icon.svg`
- fail the build with an error stating that the icon must be in the AppDir root

### actual result

symlink is broken because it links to non-existent `AppDir/icon.svg`