Test failure in RelaxDeps roundtrip Quickcheck tests

I saw the failure in https://travis-ci.org/haskell-pushbot/cabal-binaries/builds/456508583#L1376, but I can also reproduce it on master:

```
$ ./dist-newstyle/build/x86_64-linux/ghc-8.2.2/cabal-install-2.5.0.0/t/unit-tests/build/unit-tests/unit-tests --pattern=RelaxDeps --quickcheck-replay=21672
File modification time resolution calibration completed, maximum delay
observed: 31.027 ms. Will be using delay of 62.054 for test runs.
Unit Tests
UnitTests.Distribution.Client.ProjectConfig
individual parser tests
RelaxDeps: FAIL
*** Failed! Falsifiable (after 72 tests):
RelaxDepsSome [RelaxedDep (RelaxDepScopePackageId (PackageIdentifier {pkgName = PackageName "2vQ-ZkcSY", pkgVersion = mkVersion [1,1]})) RelaxDepModCaret (RelaxDepSubjectPkg (PackageName "D-yCu")),RelaxedDep (RelaxDepScopePackage (PackageName "all")) RelaxDepModCaret RelaxDepSubjectAll]
Use --quickcheck-replay=21672 to reproduce.
RelaxDeps': FAIL
*** Failed! Falsifiable (after 72 tests):
RelaxDepsSome [RelaxedDep (RelaxDepScopePackageId (PackageIdentifier {pkgName = PackageName "2vQ-ZkcSY", pkgVersion = mkVersion [1,1]})) RelaxDepModCaret (RelaxDepSubjectPkg (PackageName "D-yCu")),RelaxedDep (RelaxDepScopePackage (PackageName "all")) RelaxDepModCaret RelaxDepSubjectAll]
Use --quickcheck-replay=21672 to reproduce.

2 out of 2 tests failed (0.00s)
```