fix relative URL resolution for /files

[mvidner]

Problem

1. reported by Leo Manfredi: When using a relative reference in files, agama config generate does not resolve them

  files: [{
     destination: '/usr/local/share/dummy.xml',
     url: 'dummy.xml',
     chroot: false,
  }],

• openQA link: https://progress.opensuse.org/issues/184001#note-17

2. it is because the config fails schema validation (differently in Leo's case and my test case)
3. Agama outputs a validation failure message, and the invalid config, without resolving references. (Outputting the invalid config is intentional to make debugging easy)
4. It also does not stop processing at that point. That is IMHO the real bug

Solution

Short description of the fix.

Testing

• for now just modify a test to show the bug

Screenshots

If the fix affects the UI attach some screenshots here.

Documentation

Remember to look at it from the user's perspective. Yes you have made the compiler happy.
But will the humans even know about your contribution? Sometimes they cannot miss it,
other times they need advertisement and explanation.

Look for relevant sections and adjust:

• The description parts of the JSON schema
• Is the CLI affected? See cli.md for a complete overview,
  change the /// comments (rust doc)
  and update the .md with cargo xtask markdown
• https://agama-project.github.io/ (source)
• Run: git ls-files '*.md'

[coveralls]

coverage: 64.279% (-11.9%) from 76.147%
when pulling 6458324 on relative-urlref-files
into e9b4908 on master.

[imobachgs]

The source of the problem is that the profile is not valid. The behaviour of the agama config generate project is pretty confusing, because it prints the profile and the user might think it was processed despite the error. See https://github.com/agama-project/agama/blob/master/rust/agama-cli/src/config.rs#L233.