go/doc: NewFromFiles: fix panic on Files with SkipObjectResolution
This CL fixes a panic in NewFromFiles when it is provided files
produced by the parser in SkipObjectResolution mode, which skips
the step of connecting ast.Idents to (deprecated) ast.Objects.
Instead of calling ast.NewPackage, which performs a number of
unnecessary steps, we just construct the ast.Package directly.
Fixes #66290
Change-Id: Id55bd30d8afb9d396c3901070e7607c5a22030d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/675036 Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>