Generalise the implicit prelude import
Now it's possible to have two lists of imports: * extra_imports are imports that are always added to the context * prelude_imports are imports that are added if we don't have any open modules in scope. No UI changes or new commands are added for now. This was functionality that we needed in our customized GHCi at Facebook, so I wanted to get it upstream to reduce the differences between our version and the upstream version.
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