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This change was previously part of [D4904](https://phabricator.haskell.org/D4904), but is being split off to aid in getting this reviewed and merged. * The compiler code is built with `NoImplicitPrelude`, but GHCi's modules are incompatible with it. So, this adds the pragma to all GHCi modules that didn't have it, and adds imports of Prelude. * In order to run GHC within itself, a `call of 'initGCStatistics` needed to be skipped. This uses CPP to skip it when `-DGHC_LOADED_INTO_GHCI` is set. * There is an environment variable workaround suggested by Ben Gamari [1], where `_GHC_TOP_DIR` can be used to specify GHC's top dir if `-B` isn't provided. This can be used to solve a problem where the GHC being run within GHCi attempts to look in `inplace/lib/lib/` instead of `inplace/lib/`. [1]: https://phabricator.haskell.org/D4904#135438 Reviewers: goldfire, bgamari, erikd, alpmestan Reviewed By: alpmestan Subscribers: alpmestan, lelf, rwbarton, thomie, carter Differential Revision: https://phabricator.haskell.org/D4986
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