[dev.typeparams] cmd/compile/internal/noder: refactor irgen import handling

Rather than re-parsing and re-resolving the import path string, use
the PkgName object provided by types2 to determine what package path
it refers to.

Also, decompose importfile into smaller functions, so that we can
directly pass the already-resolved package path to the importer.

Finally, switch to simply using height as calculated by types2 rather
than redoing the calculations.

Change-Id: I3338f4e68387b2835b2e58d6df65d740d6a648cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/323309


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Trust: Dan Scales <danscales@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>