Header-only devel package PROVIDES field fixes

[lawruble13]

• In the previous version, CPACK_DEBIAN_PACKAGE_PROVIDES was not evaluated, causing packages to include the literal string CPACK_DEBIAN_PACKAGE_PROVIDES in their provides field.
• Changed the behaviour to more closely match that of CMake, using CPACK_<GEN>_PACKAGE_PROVIDES only if a component-specific variable is not set.
  • The provides field for the base package name without -dev[el] is added in either case.
• The Debian and RPM provides are correctly split up to match CMake's behaviour.