env GO111MODULE=on

go list -mod=vendor example.com/...
stdout ^example.com/x$
stdout ^example.com/x/y$
! stdout ^example.com/x/vendor

-- go.mod --
module example.com/m

-- vendor/modules.txt --
# example.com/x v0.0.0
example.com/x
# example.com/x/y v0.1.0
example.com/x/y

-- vendor/example.com/x/go.mod --
module example.com/x
-- vendor/example.com/x/x.go --
package x

-- vendor/example.com/x/y/go.mod --
module example.com/x/y
-- vendor/example.com/x/y/y.go --
package y

-- vendor/example.com/x/vendor/z/z.go --
package z