cmd/go: ignore implicit imports when the -find flag is set

The documentation of the go list -find flag says that the Deps list will
be empty. However the current implementation adds implicit imports when
supporting Cgo or SWIG and when linking a main package.

Update the documentation of PackageOpts.IgnoreImport to clarify that
both explicit and implicit imports are ignored.

Add a regression test.

Fixes #46092

Change-Id: I37847528d84adb7a18eb6ff29e4af4b4318a66fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/318770


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