Add metainfo.xml support with symlink for backward compatibility

[Copilot]

Implements the symlink approach from PR #55 discussion: when only one of .appdata.xml (legacy) or .metainfo.xml (current freedesktop.org standard) exists, create a symlink to the other name. This ensures all downstream tools work regardless of which naming convention the AppDir uses.

Changes

• Check for both .appdata.xml and .metainfo.xml in usr/share/metainfo/
• If only one exists, create symlink for the missing name pointing to the existing file
• Warning messages now recommend .metainfo.xml per freedesktop.org spec
• Error messages include strerror(errno) for symlink failures

Behavior

AppDir contains	Action
Only .appdata.xml	Creates .metainfo.xml → .appdata.xml symlink
Only .metainfo.xml	Creates .appdata.xml → .metainfo.xml symlink
Both files	No action
Neither file	Warning recommends .metainfo.xml

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Create a PR that implements the idea expressed in @AppImage/appimagetool/pull/55 using a symlink from the new to the old (already supported) file. Make sure it works no matter whether we have the old or new file path to begin with.

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[Copilot]

Pull request overview

This PR adds support for the current freedesktop.org standard .metainfo.xml file naming convention while maintaining backward compatibility with the legacy .appdata.xml naming. When only one of the two file types exists in usr/share/metainfo/, the tool automatically creates a symlink to ensure both paths work, allowing downstream tools to use either naming convention.

Key changes:

• Checks for both .appdata.xml and .metainfo.xml files in the metainfo directory
• Creates relative symlinks for backward compatibility when only one file type exists
• Updates warning messages to recommend .metainfo.xml per current freedesktop.org specification
• Enhances error messages with strerror(errno) for symlink creation failures

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[probonopd]

Does this resolve the issue? Please comment. Thanks!