apidiff: refactor the first two pass for package checking

old:           new:
type A = B     type A B
type B int     type B int

The existence of the two for loop is to prevent type match
accidentally unalias the old A as B and map the old B to
the new A.

So the first loop will strictly perform exact name match
on exported non-alias type, i.e. when apidiff find a non-alias
type X, find the X in the new package.

(This first loop make sure the Unalias will be no op inside
of the checkObjects.)

Then the second loop can safely assume the name matches are
all found and loop over the rest of the exported type.

Change-Id: I6fadc2c1543da5600a89a22585344feda4ab505d
Reviewed-on: https://go-review.googlesource.com/c/exp/+/847525
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Hongxiang Jiang <hxjiang@golang.org>