apidiff: require exported old types to match same-named type in new

Previously, correspondence was sometimes established between completely
unrelated types. This CL tightens the requirements for two exported types
to correspond: the only possibilities are that they have the same name,
or the new type is an alias for the old.

Examples:
	old: type T int    new: type T int
	same name: the T's correspond

	old: type T int    new: type T = U; type U int
	different name, but T is an alias for U.

This might seem to exclude another case:

	old: type T = U; type U int		new: type U int

It seems that T should correspond to U even though the above conditions
aren't met. But in fact U corresponds to U here, not T. Correspondence
is 1:1; we pick the closest match.

This may seem unrelated to the new test case, which reports that two
struct fields are incompatible instead of just one. But the reason that
the old code didn't report one of the fields is that it established a
correspondence between the fields' types, despite their being completely
unrelated.

Fixes golang/go#81523.

Change-Id: I8ab02d13acb606d4ea26fc73fa4d7ee5042d18d7
Reviewed-on: https://go-review.googlesource.com/c/exp/+/836046
Reviewed-by: Hongxiang Jiang <hxjiang@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>