internal/cmd/deadcode: add -whylive=function flag

The -whylive flag explains why a function is live (not dead code),
by displaying an arbitrary shortest path to the function from
one of the main entrypoints. Call paths from main packages (not tests),
from main functions (not init functions), and involving only
static calls are preferred.

The output logic is unified so that the usual mode and -whylive
both compute a list of JSON objects, which are then formatted
by -json or -format=template.

Also, a test.
Also, fix the test framework to correctly handle a sequence of commands(!).

Fixes golang/go#61263

Change-Id: I6aafc851c9c57e27a0a8f0d723e20fb2f8b57ad7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/540219
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>