go/types, types2: move Checker.infer into infer.go; delete infer2.go

go/types, types2: move Checker.infer into infer.go; delete infer2.go

Adjust imports but no other code changes otherwise.

Change-Id: Iffbd7f9b1786676a42b68d91ee6cc7df07d776bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/471015
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>