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

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to reproduce

1. create an AppDir the following.desktop file:
   app.desktop:

   [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