Help: Modernize PackageConfigHelpers example

The example shown in the module documentation for
CMakePackageConfigHelpers was showing its age, by:

1. Hardcoding directory prefixes (`etc/`, `lib/`, etc.) instead of
   using GNUInstallDirs
2. Handwaving `set()` commands (incorrectly!) as:
     `set(VAR dir/ ... CACHE )`
   which should, at the very least, be:
     `set(VAR dir/ CACHE ... )`
3. Installing CMake configuration files to `lib/Foo/cmake/`, when
   current practice favors `${CMAKE_INSTALL_LIBDIR}/cmake/Foo/`

This updated example addresses all of those issues.
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