go/types, types2: consistently use error_ type for sub-errors (cleanup)

go/types, types2: consistently use error_ type for sub-errors (cleanup)

Also, rename reportAltDecl/recordAltDecl to addAltDecl and
move function into errors.go.

Change-Id: Ie5210d1989f1e51fc5fec483dfa6dba8c4212b59
Reviewed-on: https://go-review.googlesource.com/c/go/+/567616
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>