[dev.typeparams] cmd/compile: refactor import logic

This CL refactors noder's package import logic so it's easier to reuse
with types2 and gcimports. In particular, this allows the types2
integration to now support vendored packages.

Change-Id: I1fd98ad612b4683d2e1ac640839e64de1fa7324b
Reviewed-on: https://go-review.googlesource.com/c/go/+/282919


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>