go/types: walk all imports when determining package name ambiguity

CL 209578 disambiguated paths among imported packages, but as
demonstrated in #43119, formatted types may reference packages that are
not directly imported.

Fix this by recursively walking all imports to determine whether there
is any ambiguity in the import graph. This might result in
over-qualification of names, but it is straightforward and should
eliminate any ambiguity.

In general this should be fine, but might introduce risk of infinite
recursion in the case of an importer bug, or performance problems for
very large import graphs. Mitigate the former by tracking seen packages,
and the latter by only walking the import graph once an error has been
produced.

Fixes #43119

Change-Id: If874f050ad0e808db8e354c2ffc88bc6d64fd277
Reviewed-on: https://go-review.googlesource.com/c/go/+/313035


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>