Further work refactoring and enhancing GHCi will make it desirable to
split up GHCi's code-base into multiple modules with specific functions,
and rather than have several top-level 'Ghci*' modules, it's nicer to
have a common namespace. This commit is provides the basis for that.

Note that the remaining GHCi.* namespace belongs to the new `ghci`
package.

Differential Revision: https://phabricator.haskell.org/D1593