add minimal reproducible example

.gitignore

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+dist-newstyle/

cabal.project

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+packages:
+  src/a
+  src/b

src/a/a.cabal

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+cabal-version: 3.6
+name:          a
+version:       1
+
+library
+    exposed-modules:  A
+    build-depends:    base
+    hs-source-dirs:   src
+    default-language: Haskell2010
+    ghc-options:      -Wall -fhide-source-paths
+
+test-suite a-tests
+    type:             exitcode-stdio-1.0
+    main-is:          Tests.hs
+    build-depends:
+        , a
+        , base
+        , hspec
+
+    hs-source-dirs:   unit-test
+    default-language: Haskell2010
+    ghc-options:      -Wall -fhide-source-paths

src/a/src/A.hs

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+module A
+    ( one
+    ) where
+
+one :: Int
+one = 1

src/a/unit-test/Tests.hs

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+import           Test.Hspec
+
+main :: IO ()
+main = hspec spec
+
+spec :: Spec
+spec = describe "one" $ it "works" $ 1 `shouldBe` (1 :: Int)

src/b/b.cabal

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+cabal-version: 3.6
+name:          b
+version:       1
+
+library
+    exposed-modules:  B
+    build-depends:
+        , a
+        , base
+
+    hs-source-dirs:   src
+    default-language: Haskell2010
+    ghc-options:      -Wall -fhide-source-paths
+
+test-suite b-tests
+    type:             exitcode-stdio-1.0
+    hs-source-dirs:   unit-test
+    main-is:          Tests.hs
+    build-depends:
+        , base
+        , hspec
+
+    default-language: Haskell2010
+    ghc-options:      -Wall -fhide-source-paths

src/b/src/B.hs

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+module B
+    ( add
+    ) where
+
+add :: Int -> Int -> Int
+add a b = a + b

src/b/unit-test/Tests.hs

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+import           Test.Hspec
+
+main :: IO ()
+main = hspec spec
+
+spec :: Spec
+spec = describe "one" $ it "works" $ 1 `shouldBe` (1 :: Int)