PAC: correctly handle log() and diff() operators in the equation defining the target

By default, the preprocessor substitutes logs and diffs by auxiliary variables
only in equations of auxiliary models (VAR, PAC). Hence they are not
substituted in the equation defining the target, since it is outside the
auxiliary model.

The fix consists in doing the substitution in that equation as well, when we
parse the linear combination within that equation.

Ref. dynare#1837