The existing cycle detection scheme passes around a (type name)
path; when a type name re-appears in the path, a cycle is reported.
Indirections (as in *T, func(T), etc.) are broken by starting a new
(nil) path. The problem with this approach is that it doesn't work
for cycles involving alias type names since they may be invalid
even if there is an indirection. Furthermore, the path must be
passed around through all functions which is currently not the
case, which leads to less optimial error reporting.

The new code is using the previously introduced color marking
scheme and global object path for package-level cycle detection
(function-local cycle detection doesn't use the same code path
yet but is also much less important as cycles can only be created
using the type being declared).

The new code is guarded with an internal flag (useCycleMarking)
so it can be disabled in short notice if this change introduced
unexpected new issues.

Fixes #23139.
Fixes #25141.

For #18640.
For #24939.

Change-Id: I1bbf2d2d61a375cf5885b2de1df0a9819d63e5fa
Reviewed-on: https://go-review.googlesource.com/115455Reviewed-by: Alan Donovan <adonovan@google.com>