interesting. I just found this:

https://downloads.haskell.org/~ghc/7.8.4/docs/html/users_guide/interactive-evaluation.html#ghci-import-decl
2.4.5.5. :module and :load
...
You can add a module to the scope (via :module or import) only if either (a) it is loaded, or (b) it is a module from a package that GHCi knows about. Using :module or import to try bring into scope a non-loaded module may result in the message “module M is not loaded”.