The user package registry populated by the `export()` command causes
side effects outside the build and source directories. Such effects
should be opt-in rather than op-out. Introduce a policy to change
default behavior of `export(PACKAGE)` to do nothing.
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.
Robert Maynard