Wrap the library Alt_ergo_prelude

Unwrapped this library was a big mistake, mea maxima culpa.
When I added this prelude, I did know how to use `-open` with a
wrapped library.

I couldn't compile smtml with `alt-ergo-lib` because the library of
smtml depends on a module Prelude from another library.

src/dune

@@ -17,6 +17,5 @@
 (library
  (name alt_ergo_prelude)
  (package alt-ergo-lib)
- (wrapped false)
  (modules prelude)
 )

src/lib/dune

@@ -7,7 +7,7 @@
 (library
   (name AltErgoLib)
   (public_name alt-ergo-lib)
-  (ocamlopt_flags (:standard -open Prelude))
+  (flags (:standard -open Alt_ergo_prelude__Prelude))
   ; external dependencies
   (libraries
     seq