Backpack doesn't work with DuplicateRecordFields

With

{-# LANGUAGE DuplicateRecordFields #-}
unit p where
    signature A where
        data A = A { foo :: Int }
        data B = B { foo :: Bool }

I get:

[1 of 1] Processing p
  [1 of 1] Compiling A[sig]           ( p/A.hsig, nothing )

<no location info>: error:
    The identifier $sel:foo:A does not exist in the local signature.
    (Try adding it to the export list of the hsig file.)

This is happening because the "exports" test is checking only the children of AvailTC, and not the field labels. Should be an easy fix.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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