URL: unify some key definitions
URL: unify some key definitions

Use the private extern version of the constnat string macro.  This allows for
the constnat string macro to be used for defining keys which are exported.  It
also avoids having two different copies of the keys which are used independently
based on the compiler version.

JGiola/swift-corelibs-foundation,bolshedvorsky/swift-corelibs-foundation,apple/swift-corelibs-foundation,bolshedvorsky/swift-corelibs-foundation,johnno1962c/swift-corelibs-foundation,alblue/swift-corelibs-foundation,parkera/swift-corelibs-foundation,amosavian/swift-corelibs-foundation,johnno1962e/swift-corelibs-foundation,mohitathwani/swift-corelibs-foundation,johnno1962b/swift-corelibs-foundation,parkera/swift-corelibs-foundation,johnno1962c/swift-corelibs-foundation,johnno1962c/swift-corelibs-foundation,mattrajca/swift-corelibs-foundation,mattrajca/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,alblue/swift-corelibs-foundation,michael-lehew/swift-corelibs-foundation,alblue/swift-corelibs-foundation,bolshedvorsky/swift-corelibs-foundation,apple/swift-corelibs-foundation,alblue/swift-corelibs-foundation,amraboelela/swift-corelibs-foundation,e78l/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,amraboelela/swift-corelibs-foundation,mohitathwani/swift-corelibs-foundation,ikesyo/swift-corelibs-foundation,mattrajca/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,ikesyo/swift-corelibs-foundation,e78l/swift-corelibs-foundation,ikesyo/swift-corelibs-foundation,bolshedvorsky/swift-corelibs-foundation,amosavian/swift-corelibs-foundation,parkera/swift-corelibs-foundation,amosavian/swift-corelibs-foundation,johnno1962c/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,amraboelela/swift-corelibs-foundation,e78l/swift-corelibs-foundation,johnno1962e/swift-corelibs-foundation,mohitathwani/swift-corelibs-foundation,michael-lehew/swift-corelibs-foundation,johnno1962b/swift-corelibs-foundation,e78l/swift-corelibs-foundation,johnno1962b/swift-corelibs-foundation,ikesyo/swift-corelibs-foundation,johnno1962e/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation,ikesyo/swift-corelibs-foundation,michael-lehew/swift-corelibs-foundation,amosavian/swift-corelibs-foundation,michael-lehew/swift-corelibs-foundation,mohitathwani/swift-corelibs-foundation,alblue/swift-corelibs-foundation,amraboelela/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,johnno1962b/swift-corelibs-foundation,mattrajca/swift-corelibs-foundation,mohitathwani/swift-corelibs-foundation,e78l/swift-corelibs-foundation,johnno1962c/swift-corelibs-foundation,parkera/swift-corelibs-foundation,amosavian/swift-corelibs-foundation,johnno1962e/swift-corelibs-foundation,bolshedvorsky/swift-corelibs-foundation,mattrajca/swift-corelibs-foundation,johnno1962b/swift-corelibs-foundation,johnno1962e/swift-corelibs-foundation,amraboelela/swift-corelibs-foundation,michael-lehew/swift-corelibs-foundation