Rename `ExistingOnly`

[jonsequitur]

    // I'm not sold on the naming around [ExistingOnly], because
    // * Guideline: DO give methods names that are verbs or verb phrases.
    // * Guideline: CONSIDER naming the type that holds extension methods for itsfunctionality—for example, use “Routing” instead of “[ExtendedType]Extensions.”
    //
    // The only existing "ExistingOnly" in the BCL is System.Security.Cryptography.X509Certificates.OpenFlags.OpenExistingOnly, which is an enum member.
    // "AcceptExistingOnly".  "RequirePathExists"
    // Static methods should also consider their static invocation model:  ArgumentExtensions.ExistingOnly(inputFile)... that has no meaning to me.
    // ArgumentValidation.RequirePathExists(inputFile), or ArgumentRestrictions.RequirePathExists(inputFile), or something like that, does.
    public static class ArgumentExtensions 
    {