Fix monolithic inplace build tool PATH

Monolithic inplace packages have a directory for each exe, so we allow
multiple paths and return all the exe paths of the package.

Fixes haskell#5104

cabal-install/Distribution/Client/ProjectPlanning.hs

@@ -1522,7 +1522,7 @@ elaborateInstallPlan verbosity platform compiler compilerprogdb pkgConfigDB
             external_exe_map = Map.fromList $
                 [ (getComponentId pkg, path)
                 | pkg <- external_exe_dep_pkgs
-                , Just path <- [planPackageExePath pkg] ]
+                , path <- planPackageExePaths pkg ]
             exe_map1 = Map.union external_exe_map exe_map
 
             external_lib_cc_map = Map.fromListWith Map.union
@@ -1583,24 +1583,30 @@ elaborateInstallPlan verbosity platform compiler compilerprogdb pkgConfigDB
                          -> SolverId -> [ElaboratedPlanPackage]
     elaborateLibSolverId mapDep = filter (matchPlanPkg (== CLibName)) . mapDep
 
-    -- | Given an 'ElaboratedPlanPackage', return the path to where the
-    -- executable that this package represents would be installed.
-    planPackageExePath :: ElaboratedPlanPackage -> Maybe FilePath
-    planPackageExePath =
+    -- | Given an 'ElaboratedPlanPackage', return the paths to where the
+    -- executables that this package represents would be installed.
+    -- The only case where multiple paths can be returned is the inplace
+    -- monolithic package one, since there can be multiple exes and each one
+    -- has its own directory.
+    planPackageExePaths :: ElaboratedPlanPackage -> [FilePath]
+    planPackageExePaths =
         -- Pre-existing executables are assumed to be in PATH
         -- already.  In fact, this should be impossible.
-        InstallPlan.foldPlanPackage (const Nothing) $ \elab -> Just $
+        InstallPlan.foldPlanPackage (const []) $ \elab ->
             binDirectoryFor
                 distDirLayout
                 elaboratedSharedConfig
-                elab $
+                elab <$>
                     case elabPkgOrComp elab of
-                        ElabPackage _ -> ""
+                        -- Monolithic mode: all exes of the package
+                        ElabPackage _ -> unUnqualComponentName . PD.exeName
+                                     <$> PD.executables (elabPkgDescription elab)
+                        -- Per-component mode: just the selected exe
                         ElabComponent comp ->
                             case fmap Cabal.componentNameString
                                       (compComponentName comp) of
-                                Just (Just n) -> display n
-                                _ -> ""
+                                Just (Just n) -> [display n]
+                                _ -> [""]
 
     elaborateSolverToPackage :: SolverPackage UnresolvedPkgLoc
                              -> ComponentsGraph