[dev.typeparams] go/types: import lookup logic from dev.go2go

Changes from dev.go2go:
 + Remove support for pointer designation.
 + Remove support for method type parameters in missingMethod. We could
   leave this logic in, but it looked sufficiently shaky that I'd rather
   not bring in the additional complexity.
 + Remove the strictness flag parameter to assertableTo, since it isn't
   used.

Change-Id: I812b8d1c49f3b714b166f061fbb7f2e683a0ce86
Reviewed-on: https://go-review.googlesource.com/c/go/+/278333


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Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>