multi-repl: Should a target imply loading it's download closure?

In the multi-repl if you specify a single library target then it will *just* load that target. For example, specifying `lib:pkg` will just load `lib:pkg` into the repl.

If you want to everything you can use the `all` selector.

If you want to load `lib:pkg` and all its dependencies into the repl then you have to list all the dependencies (or at least the root dependencies). This can be a bit awkward as it's not clear exactly what the dependency structure is sometimes in big mono-repos. For example in `cabal` repo you have to start a repl with `lib:Cabal-syntax lib:cabal-install` in order to load the 5 library components which you care about.

Would it be better if `cabal repl --enable-multi-repl lib:pkg` loaded `lib:pkg` and all its dependencies into the repl?

Perhaps another solution is to add a new selector type which can express this property?

Start by reproducing `cabal repl --enable-multi-repl lib:pkg` and compare it with the `all` selector and explicit dependency selectors. The issue needs a decided behavior—implicit download-closure loading or a new selector—before implementation scope and completion criteria can be established.

Written by the indexing model from the issue text.