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>

src/cmd/go/testdata/script/mod_tagged_import_cycle.txt

@@ -0,0 +1,106 @@
+# Because 'go mod' subcommands ignore build constraints, they can encounter
+# package-import cycles that are not possible in an ordinary build. This test
+# verifies that such cycles are handled even when they cross module boundaries.
+
+# First, verify that the import graph depends on build tags as expected.
+go list -deps example.com/left
+stdout '^example.com/right$'
+go list -deps example.com/right
+! stdout left
+
+env GOFLAGS=-tags=mirror
+go list -deps example.com/left
+! stdout right
+go list -deps example.com/right
+stdout '^example.com/left$'
+env GOFLAGS=''
+
+# 'go mod why' should be agnostic to build tags.
+go mod why example.com/left
+stdout '^example.com/chiral$\n^example.com/left$'
+go mod why example.com/right
+stdout '^example.com/chiral$\n^example.com/right$'
+
+env GOFLAGS='-tags=mirror'
+go mod why example.com/left
+stdout '^example.com/chiral$\n^example.com/left$'
+go mod why example.com/right
+stdout '^example.com/chiral$\n^example.com/right$'
+env GOFLAGS=''
+
+# 'go mod tidy' should successfully handle the cycle.
+env GOFLAGS=-mod=readonly
+go mod tidy
+
+# 'go mod vendor' should copy in both packages without crashing.
+go mod vendor
+exists vendor/example.com/left/default.go
+exists vendor/example.com/left/mirror.go
+exists vendor/example.com/right/default.go
+exists vendor/example.com/right/mirror.go
+
+-- go.mod --
+module example.com/chiral
+
+go 1.14
+
+require (
+	example.com/left v0.1.0
+	example.com/right v0.1.0
+)
+
+replace (
+	example.com/left => ./left
+	example.com/right => ./right
+)
+-- chiral.go --
+// Package chiral imports packages in an order that depends on build tags.
+package chiral
+-- default.go --
+// +build !mirror
+
+package chiral
+
+import _ "example.com/left"
+-- mirror.go --
+// +build mirror
+
+package chiral
+
+import _ "example.com/right"
+-- left/go.mod --
+module example.com/left
+
+go 1.14
+
+require example.com/right v0.1.0
+
+replace example.com/right v0.1.0 => ../right
+-- left/default.go --
+// +build !mirror
+
+package left
+
+import _ "example.com/right"
+-- left/mirror.go --
+// +build mirror
+
+package left
+-- right/go.mod --
+module example.com/right
+
+go 1.14
+
+require example.com/left v0.1.0
+
+replace example.com/left v0.1.0 => ../left
+-- right/default.go --
+// +build !mirror
+
+package right
+-- right/mirror.go --
+// +build mirror
+
+package right
+
+import _ "example.com/left"