Add package test for --project-dir

cabal-testsuite/PackageTests/ProjectDir/app/App.hs

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+module Main where
+
+main :: IO ()
+main = pure ()

cabal-testsuite/PackageTests/ProjectDir/app/app.cabal

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+cabal-version: 3.6
+name:          app
+version:       0.1
+
+executable app
+  main-is:       App.hs
+  build-depends: base

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

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+import Test.Cabal.Prelude
+
+main :: IO ()
+main = cabalTest $ recordMode DoNotRecord $ do
+  env <- getTestEnv
+
+  let cwd = testCurrentDir env
+
+  -- Relative directory
+  cabal "v2-build" [ "--project-dir=proj", "all" ]
+
+  -- Absolute directory
+  cabal "v2-build" [ "--project-dir", (cwd </> "proj"), "all" ]
+
+  cabal "v2-clean" [ "--project-dir=proj" ]
+
+  withProjectFile "nix/cabal.project" $ do
+    cabal "v2-build" [ "--project-dir=proj", "extra" ]
+
+    cabal "v2-clean" [ "--project-dir=proj" ]
+
+  -- App with no cabal.project
+  void $ cabal_raw' [ "run", "--project-dir=app", "app" ] Nothing

cabal-testsuite/PackageTests/ProjectDir/proj/App.hs

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+module Main where
+
+main :: IO ()
+main = pure ()

cabal-testsuite/PackageTests/ProjectDir/proj/Lib.hs

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+module Lib where
+
+x :: ()
+x = ()

cabal-testsuite/PackageTests/ProjectDir/proj/cabal.project

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+packages: .
+

cabal-testsuite/PackageTests/ProjectDir/proj/nix/cabal.project

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+packages: .
+
+package proj
+  flags: +extra

cabal-testsuite/PackageTests/ProjectDir/proj/proj.cabal

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+cabal-version: 3.8
+name:          proj
+version:       0.1
+
+flag extra
+  default:     False
+  manual:      True
+
+library
+  build-depends:   base
+  exposed-modules: Lib
+
+executable extra
+  build-depends:   base
+  main-is:         App.hs
+  if !flag(extra)
+    buildable: False