Fix package shadowing order (#4072)
Later packages are supposed to shadow earlier ones in the stack, unless the ordering is overriden with -package-id flags. Unfortunately an earlier fix for something else had sorted the list of packages so that it was in lexicographic order by installedPackageId, and sadly our test (cabal/shadow) didn't pick this up because the lexicographic ordering happened to work for the test. I've now fixed the test so it tries both orderings.
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