SI-8743 Fix crasher with poly-methods annotated with @varargs

The code that generated the Java varargs forwarder was basing
things on the `ValDef-s` of the parameters of the source method.
But, their types refer to a type parameter skolems of the enclosing
method, which led to a type mismatch when typechecking the forwarder.

Instead, I've reworked the code to simply use the `DefDef`-s symbol's
info, which *doesn't* refer to skolems. This actually simplifies the
surrounding code somewhat; rather than repeated symbols in a map
we can just time travel the pre-uncurry method signatures to figure
out which params are releated.
Merge pull request #3896 from retronym/ticket/8743

SI-8743 Fix crasher with poly-methods annotated with @varargs