Imported interfaces must be completed, whether they are named or not.
The original code was collecting all types (including anonymous ones)
in the importer's typList. That list was used in the end to complete
interface types. When we introduced tracking of named types only, we
lost anonymous interfaces. Use an independent list of interface types
so the completion code is independent of which types are tracked.

Added test and factored some of the existing tests.

Fixes #20046.

Change-Id: Icd1329032aec33f96890380dd5042de3bef8cdc7
Reviewed-on: https://go-review.googlesource.com/41198Reviewed-by: Matthew Dempsky <mdempsky@google.com>