Move library code into subdirectory src/

Having it in the root directory ./ is inconvenient for some tooling
like `doctest`.

Author: andreasabel

hackage-server.cabal

@@ -138,6 +138,7 @@ common defaults
 
 library lib-server
   import: defaults
+  hs-source-dirs:   src
 
   -- TODO: move this into a 'library lib-client';
   --       needs minor source-tree restructuring