Deadcode improvements

Technical Discussion
1

deadcode from x/tools has some awkward assumptions and error UX:

  • deadcode assumes that all Go projects have an application (main package) and/or a unit test suite. But some Go projects merely implement convenience wrappers around other CLI tools, so there’s no main package, nor not much to test.
  • Even though deadcode assumes that every project has a main package and/or a unit test suite, deadcode currently disables unit test scanning by default. -test should be the default behavior. It’s reasonable to assume that every Go project has at least one unit test. If they don’t, that’s generally a code smell, and there should be a way to disable -test as opt-out rather than opt-in.
  • The error message when neither a main package nor unit test is available is confusing and misleading: deadcode: no main packages. For example, the error message should clarify that a main package and/or a unit test would also be sufficient to help it conduct a scan.