Generate a FileTarget for each possible target location

If a target file has multiple possible locations, then we
assume they are all separate file targets.
This happens with '.hs-boot' files if they are in the root directory of the project.
GHC reports options such as '-i. A' as 'TargetFile A.hs' instead of 'TargetModule A'.
In 'fromTargetId', we dutifully look for '.hs-boot' files and add them to the
targetLocations of the TargetDetails. Then we add everything to the 'knownTargetsVar'.
However, when we look for a 'Foo.hs-boot' file in 'FindImports.hs', we look for either

 * TargetFile Foo.hs-boot
 * TargetModule Foo

If we don't generate a TargetFile for each potential location, we will only have
'TargetFile Foo.hs' in the 'knownTargetsVar', thus not find 'TargetFile Foo.hs-boot'
and also not find 'TargetModule Foo'.

ghcide/session-loader/Development/IDE/Session.hs

@@ -479,12 +479,27 @@ loadSessionWithOptions recorder SessionLoadingOptions{..} dir = do
         -- files in the project so that `knownFiles` can learn about them and
         -- we can generate a complete module graph
     let extendKnownTargets newTargets = do
-          knownTargets <- forM newTargets $ \TargetDetails{..} ->
+          knownTargets <- concatForM newTargets $ \TargetDetails{..} ->
             case targetTarget of
-              TargetFile f -> pure (targetTarget, [f])
+              TargetFile _ -> do
+                -- If a target file has multiple possible locations, then we
+                -- assume they are all separate file targets.
+                -- This happens with '.hs-boot' files if they are in the root directory of the project.
+                -- GHC reports options such as '-i. A' as 'TargetFile A.hs' instead of 'TargetModule A'.
+                -- In 'fromTargetId', we dutifully look for '.hs-boot' files and add them to the
+                -- targetLocations of the TargetDetails. Then we add everything to the 'knownTargetsVar'.
+                -- However, when we look for a 'Foo.hs-boot' file in 'FindImports.hs', we look for either
+                --
+                --  * TargetFile Foo.hs-boot
+                --  * TargetModule Foo
+                --
+                -- If we don't generate a TargetFile for each potential location, we will only have
+                -- 'TargetFile Foo.hs' in the 'knownTargetsVar', thus not find 'TargetFile Foo.hs-boot'
+                -- and also not find 'TargetModule Foo'.
+                pure $ map (\f -> (TargetFile f, [f])) (nubOrd targetLocations)
               TargetModule _ -> do
                 found <- filterM (IO.doesFileExist . fromNormalizedFilePath) targetLocations
-                return (targetTarget, found)
+                return [(targetTarget, found)]
           hasUpdate <- join $ atomically $ do
             known <- readTVar knownTargetsVar
             let known' = flip mapHashed known $ \k ->