Allow fallback/backup urls in flatpak-manifests

[alexlarsson]

From @adityashah1212 on June 25, 2017 16:38

Add a link to backup or fallback url, in case the first one is not working for sources in json

Copied from original issue: flatpak/flatpak#868

[AsavarTzeth]

Today I got reminded of how relevant this feature is. SourceForge has been in disaster recovery for a great deal of time now and it is blocking me from updating an application on flathub.

I would prefer though, if this was solved on a sources level, rather than with a fallback url. That should be a great deal more flexible and reliable. For example it would allow fallbacks to use a different type.

Here is a real world example of how this could look like. The key "fallback-sources" would be a property available to all sources.

"sources": [
  {
    "type": "archive",
    "url": "https://downloads.sourceforge.net/project/asciidoc/asciidoc/8.6.9/asciidoc-8.6.9.tar.gz",
    "sha256": "78db9d0567c8ab6570a6eff7ffdf84eadd91f2dfc0a92a2d0105d323cab4e1f0",
    "fallback-sources": [
      { 
        "type": "git",
        "url": "https://github.com/asciidoc/asciidoc.git",
        "tag": "8.6.9",
        "commit": "2ff8681ecb826658c0a1466733f2ebd974aa934b"
      }
    ]
  }
]