cmd/go/internal/load: refactor setErrorPos to PackageError.setPos

Renamed setErrorPos to setPos, made it a method of PackageError,
and removed its Package parameter and return value. This makes it
more clear that setPos modifies PackageError and does not create a new
Package.

Change-Id: I26c58d3d456c7c18a5c2598e1e8e158b1e6b4b36
Reviewed-on: https://go-review.googlesource.com/c/go/+/283637


Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>