Add a test to check that extra-prog-path is honoured for local packages

Whilst fixing #10692, I realised there was also this bug where
extra-prog-path would not be honoured for specific packages.

The idea behind extra-prog-path is that each local package can use a
different version of a preprocessor if desired.

cabal-testsuite/PackageTests/ExtraProgPathLocal/cabal.project

+packages: client

cabal-testsuite/PackageTests/ExtraProgPathLocal/client/Hello.x

+This file is deliberately an invalid .x file,
+to ensure that we pick up the local alex script rather than
+any system-wide alex executable.

cabal-testsuite/PackageTests/ExtraProgPathLocal/client/client.cabal

+name:                client
+version:             0.1.0.0
+synopsis:            Checks build-tool-depends are put in PATH
+license:             BSD3
+category:            Testing
+build-type:          Simple
+cabal-version:       >=1.10
+
+executable             hello-world
+  main-is:             Hello.hs
+  build-depends:       base
+  default-language:    Haskell2010

cabal-testsuite/PackageTests/ExtraProgPathLocal/scripts/alex

+#! /usr/bin/env bash
+
+echo "I am not the alex you are looking for"

cabal-testsuite/PackageTests/ExtraProgPathLocal/scripts/alex.shim

+path = "FINDSH/sh.exe"
+args = "SCRIPTSDIR/alex"

cabal-testsuite/PackageTests/ExtraProgPathLocal/scripts2/alex

+#! /usr/bin/env bash
+
+echo "I am the alex you are looking for"
+echo "module Main where main = print ()" > $3

cabal-testsuite/PackageTests/ExtraProgPathLocal/scripts2/alex.shim

+path = "FINDSH/sh.exe"
+args = "SCRIPTS2DIR/alex"

cabal-testsuite/PackageTests/ExtraProgPathLocal/setup.out

+# cabal v2-build
+Configuration is affected by the following files:
+- cabal.project
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - client-0.1.0.0 (exe:hello-world) (first run)
+Configuring executable 'hello-world' for client-0.1.0.0...
+Preprocessing executable 'hello-world' for client-0.1.0.0...
+Building executable 'hello-world' for client-0.1.0.0...

cabal-testsuite/PackageTests/ExtraProgPathLocal/setup.test.hs

+import Test.Cabal.Prelude
+import System.Directory
+
+-- Test package-local extra-prog-path works.
+main = cabalTest $ do
+    env <- getTestEnv
+    let
+      testDir = testCurrentDir env
+      tmpDir  = testTmpDir     env
+      scripts1 = tmpDir </> "scripts"
+      scripts2 = tmpDir </> "scripts2"
+
+    -------------------------
+    -- Workaround for the fact that, on Windows, Cabal will only look for
+    -- .exe files to satisfy executable dependencs. So we have to create
+    -- shim alex.exe files (the good one in 'scripts2', the bad one in 'scripts')
+    -- with the logic below.
+    when isWindows $ do
+      mb_sh <- fmap takeDirectory <$> liftIO (findExecutable "sh")
+      case mb_sh of
+          Nothing -> skip "no sh"
+          Just sh -> do
+            let escape = concatMap (\c -> case c of '\\' -> "\\\\\\\\"; x -> [x])
+            void $ shell "sed" [ "-i", "-e", "s/FINDSH/" <> escape sh <> "/g", escape (scripts1 </> "alex.shim"), escape (scripts2 </> "alex.shim") ]
+            void $ shell "sed" [ "-i", "-e", "s/SCRIPTSDIR/" <> escape scripts1 <> "/g", escape (scripts1 </> "alex.shim") ]
+            void $ shell "sed" [ "-i", "-e", "s/SCRIPTS2DIR/" <> escape scripts2 <> "/g", escape (scripts2 </> "alex.shim") ]
+
+    -- End of Windows workarounds
+    ------------------------------
+
+    -- Add the 'scripts' directory to PATH, and add the 'scripts2' directory
+    -- to extra-prog-path.
+    --
+    -- This checks that the executables in extra-prog-path take priority over
+    -- those in PATH: 'scripts/alex' will fail, while 'scripts2/alex' will succeed.
+
+    liftIO $ appendFile (testDir </> "cabal.project") $
+      "\npackage client\n  extra-prog-path:" ++ scripts2
+    addToPath scripts1 $ cabal "v2-build" ["client"]