Fix bug on unscoped policy for exported targets

Santiago Ospina De Los Ríosrequested to merge
bugfix/cmake-option-was-always-forced-to-true into master

The flag ${ProjectName}_POLICY_UNSCOPED_EXPORTED_TARGET_VISIBILITY was set to INTERNAL which implicitly means FORCED in CMake. This means that the user never had the possibility to change this option.

This was found accidentally by @simon.praetorius in !1391 (comment 139715).

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