ocamlPackages.utop: fix environment variables

In the wrapper scripts, both OCAMLPATH and CAML_LD_LIBRARY_PATH where
being created with a trailing literal $OCAMLPATH, rather than the
expanded version. Thus if, for example, ocamlPackages.core was present
in OCAMLPATH prior to running utop, the wrapper script would set the
variable to $utop_dependencies:'$OCAMLPATH', and when using utop to
open Core.Std, the following error was reported:

    findlib: [WARNING] cannot read directory $OCAMLPATH: No such file
    or directory

This patch fixes the quoting issue, and further refactors the build to
use standard wrapProgram helper, and uses an "inner derivation" to
re-use the setupHook machinery of buildOCaml and findlib instead of
manually specifying the OCAMLPATH required for utop along with
transitive dependencies.