cmd/go: add a regression test for package import cycles guarded by build tags

I've been thinking about the relationship between the package import
graph and the module import graph, and realized that the package
import graph is not always acyclic. (The package import graph must be
acyclic given a specific set of build tags, but the 'mod' subcommands
intentionally ignore build tags.)

I'm not sure whether we have any existing regression tests that cover
this sort of cycle, so I'm adding one now. Thankfully, it passes!

Updates #36460

Change-Id: I7679320994ee169855241efa51cd45f71315f7f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/217557
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

0 files changed, 0 insertions, 0 deletions