go/types: remove objSet type in favor of explicit map type (cleanup)

Avoid confusion between (now gone) objSet and objset types.
Also: rename visited -> seen in initorder.go.

No functional changes.

Change-Id: Ib0aa25e006eee55a79a739194d0d26190354a9f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/198044


Reviewed-by: Matthew Dempsky <mdempsky@google.com>