[dev.typeparams] go/types: port lazy import resolution from types2

[dev.typeparams] go/types: port lazy import resolution from types2

This is a straightforward port of CL 323569 to go/types. It is
line-for-line identical, except where names are unexported to preserve
the current go/types API.

Change-Id: I4c78211bff90f982ca2e90ed224946716118ee31
Reviewed-on: https://go-review.googlesource.com/c/go/+/334893
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>