Ambiguous interface errors in GHCi, even with -XPackageImports

Originally reported in https://github.com/haskell/cabal/issues/4253, but I moved it here after being reassured that this was a GHCi bug, not a Cabal one.

To reproduce this issue, you'll need to install two libraries that export two constructors with the same name. The original issue uses GLFW and GLFW-b as examples, but for the puposes of test case minimalization, I've prepared a repo here (https://github.com/RyanGlScott/cabal-gh4253) with as small of an example as I can manage. To reproduce the issue, do the following:

$ git clone https://github.com/RyanGlScott/cabal-gh4253
$ cd cabal-gh4253/
$ cabal install package1/ package2/
$ ghci -XPackageImports
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
λ> import "package1" DuplicateModuleName
λ> Window

<interactive>:1:1: error:
    Ambiguous interface for ‘DuplicateModuleName’:
      it was found in multiple packages:
      package1-0.1.0.0 package2-0.1.0.0

This seems to be specific to GHCi, because I cannot find a way to trigger the same issue with ghc --make.

I've reproduced this with GHC 8.0.2 and HEAD.

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
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CC	ezyang
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