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Make purl type identifiers constants

These are fixed spec identifiers, but as package-level vars any consumer
could reassign them and change parsing behaviour process-wide.

Leaving them untyped keeps this backwards compatible: every read-only
use still compiles, and assigning to a named string type now works too.
Only taking their address or reassigning them breaks, which is the point.

KnownTypes and CandidateTypes stay vars, since maps cannot be constants.