CPack/NSIS: Fix reinstall and multiple install issues when using components.
Fix NSIS template to more thoroughly use CPACK_PACKAGE_INSTALL_REGISTRY_KEY. This allows different versions of software to have a separate sections in the registry to keep track of things (installed components, and uninstall stuff). Change default of CPACK_PACKAGE_INSTALL_REGISTRY_KEY to follow the value of CPACK_PACKAGE_INSTALL_DIRECTORY so if an installation overwrites another installation, the proper registry entries are more likely to be overwritten. Fix CPack/NSIS generator to not insert code in the NSIS template to skip installation of already installed components. This enables a repair like behavior and also enables installing patch releases on top of an older installation.
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