[Go] skip validator import if oneOf discriminator lookup is enabled

[ctreatma]

The Go generator produces invalid code for oneOf models that have discriminators when useOneOfDiscriminatorLookup is set to true. In this case, the model imports gopkg.in/validator.v2 but does not use it because validation is skipped when discriminator lookup is enabled.

This updates the Go generator so that the validator import is skipped if discriminator lookup is enabled. In order to make this change I had to move the validator import from AbstractGoCodegen to GoClientCodegen; this is a safe change because the oneOf validation code only exists in the Go client generator templates, not in the other Go generators.

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@wing328