Add Backpack Fail5 test: reproduce opaque "broken package" error

Add a test case (Backpack/Fail5) that reproduces the real-world scenario
from prrrrrrrrr where an indefinite Backpack library installed separately
(e.g. via nix callCabal2nix) causes an opaque "broken package" error
when a consumer tries to configure.

The test installs an indefinite framework package (with App signature and
a framework-utils sub-library) and a separate app-impl package, then
tries to configure a consumer that depends on both. The linker correctly
resolves App=app-impl:App, but the instantiated variant
framework+<hash> was never built, triggering:

  "broken due to missing package framework-0.1.0.0+95RTb42ZWxa9J13cUStM0q"

The error shows only an opaque hash — no hint about which signature
instantiation is missing or how to fix it. The test documents this
deficiency with assertOutputDoesNotContain, to be flipped when the
error message is improved.

Prompt: Implement the plan to reproduce prrrrrrrrr's Backpack "broken package"
error in cabal test suite

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jappeace-sloth

cabal-testsuite/PackageTests/Backpack/Fail5/Fail5.cabal

@@ -0,0 +1,32 @@
+cabal-version: 3.0
+name:          Fail5
+version:       0.1.0.0
+build-type:    Simple
+
+library framework-utils
+  exposed-modules:  Utils
+  hs-source-dirs:   repo/framework-0.1.0.0/src-utils
+  build-depends:    base
+  default-language: Haskell2010
+  visibility:       public
+
+library framework
+  signatures:       App
+  exposed-modules:  Framework
+  hs-source-dirs:   repo/framework-0.1.0.0/src repo/framework-0.1.0.0/src-sig
+  build-depends:    base, Fail5:framework-utils
+  default-language: Haskell2010
+  visibility:       public
+
+library app-impl
+  exposed-modules:  App
+  hs-source-dirs:   repo/app-impl-0.1.0.0
+  build-depends:    base
+  default-language: Haskell2010
+  visibility:       public
+
+executable consumer
+  main-is:          Main.hs
+  build-depends:    base, Fail5:framework, Fail5:framework-utils, Fail5:app-impl
+  hs-source-dirs:   repo/consumer-0.1.0.0
+  default-language: Haskell2010

cabal-testsuite/PackageTests/Backpack/Fail5/repo/app-impl-0.1.0.0/App.hs

@@ -0,0 +1,3 @@
+module App where
+appMessage :: String
+appMessage = "Hello from app-impl"

cabal-testsuite/PackageTests/Backpack/Fail5/repo/app-impl-0.1.0.0/app-impl.cabal

@@ -0,0 +1,9 @@
+cabal-version: 3.0
+name:          app-impl
+version:       0.1.0.0
+build-type:    Simple
+
+library
+  exposed-modules:  App
+  build-depends:    base
+  default-language: Haskell2010

cabal-testsuite/PackageTests/Backpack/Fail5/repo/consumer-0.1.0.0/Main.hs

@@ -0,0 +1,4 @@
+import Framework (greet)
+import Utils (utilHelper)
+main :: IO ()
+main = putStrLn greet >> putStrLn (utilHelper "direct")

cabal-testsuite/PackageTests/Backpack/Fail5/repo/consumer-0.1.0.0/consumer.cabal

@@ -0,0 +1,9 @@
+cabal-version: 3.0
+name:          consumer
+version:       0.1.0.0
+build-type:    Simple
+
+executable consumer
+  main-is:          Main.hs
+  build-depends:    base, framework, framework:framework-utils, app-impl
+  default-language: Haskell2010

cabal-testsuite/PackageTests/Backpack/Fail5/repo/framework-0.1.0.0/framework.cabal

@@ -0,0 +1,18 @@
+cabal-version: 3.0
+name:          framework
+version:       0.1.0.0
+build-type:    Simple
+
+library framework-utils
+  exposed-modules:  Utils
+  hs-source-dirs:   src-utils
+  build-depends:    base
+  default-language: Haskell2010
+  visibility:       public
+
+library
+  signatures:       App
+  exposed-modules:  Framework
+  hs-source-dirs:   src src-sig
+  build-depends:    base, framework:framework-utils
+  default-language: Haskell2010

cabal-testsuite/PackageTests/Backpack/Fail5/repo/framework-0.1.0.0/src-sig/App.hsig

@@ -0,0 +1,2 @@
+signature App where
+appMessage :: String

cabal-testsuite/PackageTests/Backpack/Fail5/repo/framework-0.1.0.0/src-utils/Utils.hs

@@ -0,0 +1,3 @@
+module Utils where
+utilHelper :: String -> String
+utilHelper msg = "Framework: " ++ msg

cabal-testsuite/PackageTests/Backpack/Fail5/repo/framework-0.1.0.0/src/Framework.hs

@@ -0,0 +1,5 @@
+module Framework where
+import App (appMessage)
+import Utils (utilHelper)
+greet :: String
+greet = utilHelper appMessage