go/doc: link to struct fields in the same package

go/doc: link to struct fields in the same package

This collects up any struct direct field references
for linking. It does not attempt to do any validation
of embedded field conflicts as in CL 510315.

For #61394

Change-Id: I57dc0a0e8a71ce0bcb4e6c0963f459f76a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/729980
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>