Introduces support for instantiating packages from remote project links

[hennevogel]

Instead of returning nil for packages that might come from an remote project link, check if they are known to the backend and return an in-memory, read only Package object.

[hennevogel]

TODO:

• Project#show view should list the packages in an "Inherited Packages" tab somehow...
• Package#show view The "derived packages" information is showing something weird...
• Package#repositores view has no indicator that it's read only
• Package#revisions view: Links to links (home:hennevogel/cloc links to home:hennevogel/cloc on OBS which is a link to Documentation:Tools/cloc on OBS) show "No commits exists yet."
• Package#users view has no indicator that it's read only
• Package#attributes view has no indicator that it's read only
• Package#meta view has no indicator that it's read only
• Branching a package leads to a weird package because the BackendPackage does not consider linked information from the backend
• Requests from a readonly package (like obs-test#10) don't show a package link
  
• Request Role Addition side-link does not work. Additionally it is shown to people who have write access to the project.
• "Request Deletion" side-link does not work either. Additionally it is shown to people who have write access to the project.
• "Report this package" side-link is broken too. Additionally it is shown to people who have write access to the project.
• Watchlist functionality does not work for the package

[hennevogel]

Do not merge this.

It needs some more sophisticated authorization setup. We probably want to switch the default of the option and then explicitly enable this per Package.get_by_project_and_name call and figure out problems first....