Merge nested polytypes in more cases in resolveOverloaded

[griggt]

In some cases while going deeper into alternatives during overload resolution,
we may end up with a nested polytype after dropping contextual parameters.

In particular this can happen for an extension with a using clause, as seen
in tests/pos/i11713.scala and tests/pos/i13668.scala.

The overload applicability test fails here unless the type parameter lists
are merged.

Co-authored-by: Gagandeep Kalra gagandeepkalra1994@gmail.com
Co-authored-by: Mark T. Kennedy mtk@acm.org

Fixes #11713
Fixes #13668

[griggt]

Before this PR, we could see these failures in overload resolution:

[... snip ...]
resolve mapped: [T1](using x$2: Numeric[T1])(y: String): Nothing, [T1](using x$2: Numeric[T1]): [T2](y: T2)(using x$4: Numeric[T2]): Nothing with [applied to (200) returning ?]
resolve mapped: [T1](y: String): Nothing, [T1] => [T2](y: T2)(using x$4: Numeric[T2]): Nothing with [applied to (200) returning ?]
narrowed by size: def combine[T1](y: String): Nothing, def combine[T1]: [T2](y: T2)(using x$4: Numeric[T2]): Nothing
narrowed by shape: def combine[T1](y: String): Nothing, def combine[T1]: [T2](y: T2)(using x$4: Numeric[T2]): Nothing
narrowed by size: def combine[T1](y: String): Nothing, def combine[T1]: [T2](y: T2)(using x$4: Numeric[T2]): Nothing
narrowed by shape: def combine[T1](y: String): Nothing, def combine[T1]: [T2](y: T2)(using x$4: Numeric[T2]): Nothing
-- [E008] Not Found Error: tests/pending/i11713.scala:5:14 ---------------------
5 |val res = 100.combine(200)
  |          ^^^^^^^^^^^
  |      value combine is not a member of Int.
  |      An extension method was tried, but could not be fully constructed:
  |
  |          combine()    failed with
  |
  |              value combine: <overloaded combine> does not take parameters

[... snip ...]
resolve mapped: [L](using x$2: Candidate[L])(rhs: Bear): Unit, [L](using x$2: Candidate[L]): [RW](rhs: Fuzzy[RW]): Unit with [applied to (Fuzzy[1.type]()) returning ?]
resolve mapped: [L](rhs: Bear): Unit, [L] => [RW](rhs: Fuzzy[RW]): Unit with [applied to (Fuzzy[1.type]()) returning ?]
narrowed by size: def +[L](rhs: Bear): Unit, def +[L]: [RW](rhs: Fuzzy[RW]): Unit
narrowed by shape: def +[L](rhs: Bear): Unit, def +[L]: [RW](rhs: Fuzzy[RW]): Unit
narrowed by size: def +[L](rhs: Bear): Unit, def +[L]: [RW](rhs: Fuzzy[RW]): Unit
narrowed by shape: def +[L](rhs: Bear): Unit, def +[L]: [RW](rhs: Fuzzy[RW]): Unit
-- [E008] Not Found Error: tests/pending/i13668.scala:15:21 --------------------
15 |val fails = MyType() + Fuzzy[1]()
   |            ^^^^^^^^^^
   |      value + is not a member of MyType.
   |      An extension method was tried, but could not be fully constructed:
   |
   |          +()    failed with
   |
   |              value +: <overloaded +> does not take parameters

[odersky]

A one line change fixes two issues. Nice!