Flag for fully offline building

**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.