Move GHCi into main library in prep for !1410

Perhaps helps with #14335 too.

So just for some background, this a continuation of a line of work !891 (closed) kicked off fixing this by cleanly separating GHCI vs external interpreter vs internal interpreter in CPP, and then I in !1390 (closed) simplified the CPP by always building the external interpreter. The next step, !1410, is always building GHCi so the only CPP left is whether to support the internal interpreter: just like external interpreter support, GHCi support in an of itself doesn't incur bootstrapping or dependency problems, so I don't think the maintenance burden of CPP is worth it.

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