x/tools/internal/cmd/deadcode: add -why=symbol flag to report path from root

[adonovan]

@thanm suggests: "If you are looking for ways to improve it, here is a feature request: allow a user to pass in a symbol (ex: "k8s.io/kubernetes/pkg/scheduler/framework.NewCycleState") and have the tool print the sequence of symbols that led the target to be marked live."

[gopherbot]

Change https://go.dev/cl/540219 mentions this issue: internal/cmd/deadcode: add -whylive=function flag

[adonovan]

Example output on the function mentioned above:

kubernetes$ deadcode -whylive=k8s.io/kubernetes/pkg/scheduler/framework.NewCycleState  ./cmd/... 
			k8s.io/kubernetes/cmd/kubectl-convert.main
 ---static-L0041-->	k8s.io/component-base/cli.Run
 ---static-L0072-->	fmt.Fprintf
 --dynamic-L0225-->	(net/http.checkConnErrorWriter).Write
 --dynamic-L3602-->	k8s.io/client-go/util/workqueue.ParallelizeUntil$1
 --dynamic-L0090-->	(*k8s.io/kubernetes/pkg/scheduler/framework/preemption.Evaluator).DryRunPreemption$1
 ---static-L0557-->	(*k8s.io/kubernetes/pkg/scheduler/framework.CycleState).Clone
 ---static-L0080-->	k8s.io/kubernetes/pkg/scheduler/framework.NewCycleState

[thanm]

This looks great, thank you!