Backpack doesn't work with CPP

Steps to repro:

1. Create a package with a signature and a module which imports it
2. Add {-# LANGUAGE CPP #-} to the module

Expected result: it works

Actual result:

<command line>: unknown package: hole

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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