Hadrian doesn't rebuild changed `CoreUtils.hs`

When compiling stage2 with Hadrian, freezing stage1 causes changes to CoreUtils.hs to be ignored, and CoreUtils.hs not being recompiled.

The following steps should reproduce the problem:

1. Make a debug build with Hadrian: ./hadrian/build.sh --flavour=Devel2
2. Rebuild with freeze1: ./hadrian/build.sh --flavour=Devel2 --freeze1
3. Edit compiler/coreSyn/CoreUtils.hs, adding some change that is easily detected in the output
4. Rebuild with freeze1: ./hadrian/build.sh --flavour=Devel2 --freeze1
5. Observe how CoreUtils does not appear in the list of modules Hadrian is compiling, and how changes to that file do not end up in the stage2 compiler.

Trac metadata

Trac field	Value
Version	8.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System
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