SI-8197 Only consider immediate params for defaults, overloading

A recent commit fixed the behaviour of overloading with regards
to discarding candiates that include default arguments. The old
check was checking the wrong flag.

But, the new code is actually checking all parameter lists for
defaults, which led to a regression in scala-io, which is distilled
in the enclosed test case. Applications are typechecked one
parameter list at a time, so a holistic check for defaults doesn't
seem to make sense; only defaults in the first parameter list
ought to count.
Merge pull request #3572 from retronym/ticket/8197

SI-8197 Only consider immediate params for defaults, overloading