--abi-hash with Backpack incorrectly loads modules from dependent packages

Repro at: https://github.com/alexbiehl/cabal-backpack-register-repro

If you use Backpack with data families, you might fail during --abi-hash with:

cabal:
'/nix/store/m338klajhqlw7v4jd61fiqd82wx305fj-ghc-8.4.3-with-packages/bin/ghc'
exited with an error:
Failed to load interface for ‘Stuff’
no unit id matching ‘backpack-trans-0.1.0.0-L6CFTQZAAWWFpCQD2NXR4W-indef’ was
found

Edited Mar 10, 2019 by Edward Z. Yang

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