Tests: Improve FindPackageTest exported package version

Add a random version component so that the test is unlikely to conflict
with other tests of the same CMake version on the same machine.
Tests: Improve FindPackageTest for in-source builds

Use a different directory name for the Exporter build tree so that it is
out-of-source even when the main test is in-source.
Allow the Package Registry to be disabled (#14849)

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.
Help: Add notes for topic 'package-disable-registry'
Merge topic 'package-disable-registry'

ba387cb8 Help: Add notes for topic 'package-disable-registry'
be8ae960 Allow the Package Registry to be disabled (#14849)
d09fda5d Tests: Improve FindPackageTest for in-source builds
ac24a1c0 Tests: Improve FindPackageTest exported package version