* Divest api/ package of deps elsewhere in the nomad repo.

This will allow making api/ a module without then pulling in the
external repo, leading to a package name conflict.

This required some migration of tests to an apitests/ folder (can be
moved anywhere as it has no deps on it). It also required some
duplication of code, notably some test helpers from api/ -> apitests/
and part (but not all) of testutil/ -> api/testutil/.

Once there's more separation and an e.g. sdk/ folder those can be
removed in favor of a dep on the sdk/ folder, provided the sdk/ folder
doesn't depend on api/ or /.

* Also remove consul dep from api/ package

* Fix stupid linters

* Some restructuring

Reverting from #3963

https://www.merriam-webster.com/dictionary/existent

api.AgentSelf mirrors command.agent.agentSelf, and makes it easier to
work with the results of a call to Agent.Self()