-package P was loading all versions of P in GHCi (#7030)

-package P means "the latest version of P" if multiple versions are
installed.  It was working as advertised, but we were
eagerly *linking* all versions of P, which might cause an error if the
package has some C code, because we can't link multiple instances of
the same symbol.

MERGED from commit 62164cf5

compiler/main/Packages.lhs

@@ -776,7 +776,9 @@ mkPackageState dflags pkgs0 preload0 this_package = do
   --
   let preload1 = [ installedPackageId p | f <- flags, p <- get_exposed f ]
 
-      get_exposed (ExposePackage   s) = filter (matchingStr s) pkgs2
+      get_exposed (ExposePackage   s)
+         = take 1 $ sortByVersion (filter (matchingStr s) pkgs2)
+         --  -package P means "the latest version of P" (#7030)
       get_exposed (ExposePackageId s) = filter (matchingId  s) pkgs2
       get_exposed _                   = []