Flag for fully offline building

[asivitz]

Describe the feature request
Change --offline flag to prevent downloading an imported cabal.config. OR add another flag which does so.

Additional context
Currently there is a --offline flag that prevents downloading packages. But what about an import line in a cabal.project file? Let's say I import a particular stackage release cabal.config file. That config doesn't change, and yet every time cabal builds it will try to download it (presumable to check if there are changes). When stackage is down, there's no way to build my package (aside from editing the import out of my config).

If I pass the --offline flag I'm certainly surprised to see it fail on a connection to the internet. But there should be a flag at least that really is fully offline.

[ulysses4ever]

This is a twist on a long-standing issue, see

• --offline does not stop Cabal from attempting to access the network #10288
• "active-repositories: none" reportedly broken #10504
• cabal build --offline tries to access the network anyway #8752