Imported node generation bug fix

[lorchrob]

Stop duplicating type declarations when generating imported nodes.

Note: LGI.gen_imp_nodes processes type and constant declarations included in const_inlined_type_and_consts, but these declarations should not be included in the result.

[lorchrob]

The bug is exhibited by the following lustre file:

const C: int;
type UserType = subtype { x: int | x < C and x > 0 }; 

node N(x: UserType) returns (y: UserType)
let
    y = x;
tel

when calling with --enable CONTRACTCK