cmake: populate package registry by default

Sergiu Deitschrequested to merge
(removed):cmake-set-policy-cmp0090 into master

What does this implement/fix?

With CMake 3.15 and above the export command does not populate the package registry by default unless the CMAKE_EXPORT_PACKAGE_REGISTRY variable is set. For backwards compatibility, prefer the old behavior (even though the old one occasionally causes some confusion and is considered deprecated.)

Edited by Sergiu Deitsch

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