Add a negative test for multilibs

cabal-testsuite/PackageTests/MultipleLibraries/cabal.out

+# cabal v2-build
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - d-0.1.0.0 (lib:privatelib) (first run)
+ - p-0.1.0.0 (lib) (first run)
+Configuring library 'privatelib' for d-0.1.0.0..
+Preprocessing library 'privatelib' for d-0.1.0.0..
+Building library 'privatelib' for d-0.1.0.0..
+Configuring library for p-0.1.0.0..
+cabal: Encountered missing or private dependencies:
+    d : {privatelib} ==0.1.0.0

cabal-testsuite/PackageTests/MultipleLibraries/cabal.project

+packages:
+  d
+  p
+

cabal-testsuite/PackageTests/MultipleLibraries/cabal.test.hs

+import Test.Cabal.Prelude
+main = cabalTest $
+    void $ fails (cabal' "v2-build" ["p"])
+

cabal-testsuite/PackageTests/MultipleLibraries/d/d.cabal

+cabal-version: 3.0
+name: d
+version: 0.1.0.0
+
+-- See issue #6038
+library
+  default-language: Haskell2010
+
+library privatelib
+  visibility: private
+  default-language: Haskell2010
+

cabal-testsuite/PackageTests/MultipleLibraries/p/p.cabal

+cabal-version: 3.0
+name: p
+version: 0.1.0.0
+
+library
+  build-depends: d:privatelib
+  default-language: Haskell2010
+