Add support for the multi unit argument syntax introduced in GHC 9.4: h…

…ttps://downloads.haskell.org/ghc/9.4.4/docs/users_guide/using.html#multiple-home-units

We now support arguments of the form
```
-unit @unitA -unit @unitb
```

where the response files `unitA` and `unitB` contain the actual list of arguments for that unit:

```
-this-unit-id a-0.1.0.0
-i
-isrc
A1
A2
```

Also refactor the session loader and simplify it.

Also adds error messages on GHC 9.4 if the units are not closed (#3422).

ghcide/ghcide.cabal

@@ -165,6 +165,7 @@ library
         Development.IDE.Core.UseStale
         Development.IDE.GHC.Compat
         Development.IDE.GHC.Compat.Core
+        Development.IDE.GHC.Compat.CmdLine
         Development.IDE.GHC.Compat.Env
         Development.IDE.GHC.Compat.Iface
         Development.IDE.GHC.Compat.Logger