SI-6111 accept single-subpattern unapply pattern

An extractor pattern `X(p)` should type check for any `X.unapply`/`X.unapplySeq`
that returns an `Option[_]` -- previously we were confused about the case where
it was an `Option[(T1, ... , Tn)]`. In this case, the expected type for the
pattern `p` is simply `(T1, ... , Tn)`.

While I was at it, tried to clean up unapplyTypeList and friends (by replacing them by extractorFormalTypes).

From the spec:
   8.1.8 ExtractorPatterns

   An extractor pattern x(p1, ..., pn) where n ≥ 0 is of the same syntactic form as a constructor pattern.
   However, instead of a case class, the stable identifier x denotes an object which has a member method named unapply or unapplySeq that matches the pattern.
   An unapply method in an object x matches the pattern x(p1, ..., pn) if it takes exactly one argument and one of the following applies:

   n = 0 and unapply’s result type is Boolean.

   n = 1 and unapply’s result type is Option[T], for some type T.
     the (only) argument pattern p1 is typed in turn with expected type T

   n > 1 and unapply’s result type is Option[(T1, ..., Tn)], for some types T1, ..., Tn.
     the argument patterns p1, ..., pn are typed in turn with expected types T1, ..., Tn

1 files changed, 1 insertions, 7 deletions

diff --git a/test/files/neg/t997.check b/test/files/neg/t997.check
index c9fe0de756..186095f44a 100644
--- a/test/files/neg/t997.check
+++ b/test/files/neg/t997.check
@@ -1,13 +1,7 @@
-t997.scala:7: error: wrong number of arguments for object Foo
-"x" match { case Foo(a) => Console.println(a) }
-                    ^
-t997.scala:7: error: not found: value a
-"x" match { case Foo(a) => Console.println(a) }
-                                           ^
 t997.scala:13: error: wrong number of arguments for object Foo
 "x" match { case Foo(a, b, c) => Console.println((a,b,c)) }
                     ^
 t997.scala:13: error: not found: value a
 "x" match { case Foo(a, b, c) => Console.println((a,b,c)) }
                                                   ^
-four errors found
+two errors found