Handle recursive fieldpropagators

[mlevesquedion]

The fieldpropagator analyzer currently does not identify fieldpropagators that get their fields from another fieldpropagator, e.g. :

type Source struct { secret string }

func (s Source) Secret() string {
    return s.secret
}

func (s Source) FormattedSecret() string {
    return "the secret is: " + s.Secret()
}

The above example is admittedly a bit frivolous. I'm not sure how common these are.

The analysis is fact-based, so if a method has already been analyzed, its status as a fieldpropagator can be checked by calling pass.ImportObjectFact(obj, &isFieldPropagator{}).