When a project is packaged for redistribution the local package
registries should not be updated or consulted. They are for developers.
Add variables to disable use of package registries globally:
* CMAKE_EXPORT_NO_PACKAGE_REGISTRY that disables the export(PACKAGE)
command
* CMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY that disables the User Package
Registry in all the find_package calls.
* CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY that disables the
System Package Registry in all the find_package calls.
Update documentation and unit tests.
There is not really any need to. Downstreams can either rely on it
being provided by CMake, or copy and distribute it.
Change the documented include for the find_dependency macro.