cmd/go/internal/load: convert two global flags to an options struct

PackageOpts is a new struct type accepted by package loading
functions. It initially has two fields: IgnoreImports, and
ModResolveTests. Previously, these were global variables set by
clients. We'll add more to this in the future.

For #40775

Change-Id: I6956e56502de836d3815ce788bdf16fc5f3e5338
Reviewed-on: https://go-review.googlesource.com/c/go/+/310669


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>