[dev.typealias] cmd/compile, go/types, go/importer: various alias related fixes

cmd/compile:
- remove crud from prior alias implementation
- better comments in places

go/types:
- fix TypeName.IsAlias predicate
- more tests

go/importer (go/internal/gcimporter15):
- handle "@" format for anonymous fields using aliases
  (currently tested indirectly via x/tools/gcimporter15 tests)

For #18130.

Change-Id: I23a6d4e3a4c2a5c1ae589513da73fde7cad5f386
Reviewed-on: https://go-review.googlesource.com/35101


Reviewed-by: Alan Donovan <adonovan@google.com>