Where to hold PL/SQL constants?

I'd have a concern about having "one package to rule the constants" because package state -- constants, variables, and code -- get cached in the user's PGA at first invocation of any public variable or package. A package constant, if public, should be scoped to the package, and used by only by the methods of the package.

A constant whose scope spans packages should be in a code table with a description, joined in as required. Constant's aren't and variables don't, after all. Having a key-value-pair table of "constants" makes them all public, and makes changing them dynamically possible.