Added CMAKE_PREFIX_PATH to the module file

... to unsure that CMake projects find the available MPI enabled
packages, libraries, executables, etc.

The motivation is that CMake projects will search first in the
CMAKE_PREFIX_PATH, both for cmake specific files like <Package>Config.cmake as
well as libraries and header files in general. Adding this variable is crucial
to making sure that the MPI versions take precedence over the serial versions.

(cherry picked from commit a9297c32c6)

0004-Make-mpich.module-useful.patch

@@ -42,3 +42,4 @@ index 3d6b4fa623..b8283274ee 100644
 +prepend-path  LD_LIBRARY_PATH @libdir@
 +prepend-path  MANPATH       :@mandir@
 +prepend-path  PKG_CONFIG_PATH @LIBDIR@/lib/pkgconfig
++prepend-path  CMAKE_PREFIX_PATH @LIBDIR@