This PR switches the Nomad repository from using govendor to Go modules
for managing dependencies. Aspects of the Nomad workflow remain pretty
much the same. The usual Makefile targets should continue to work as
they always did. The API submodule simply defers to the parent Nomad
version on the repository, keeping the semantics of API versioning that
currently exists.
Pick up a panic fix of f1bd0923b8
Some CirleCI builds fail somewhat mysteriously, e.g. https://circleci.com/gh/hashicorp/nomad/52676 , due to this bug. The test json file emit the following:
```
{"Time":"2020-03-28T12:03:08.602544522Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"panic: send on closed channel\n"}
{"Time":"2020-03-28T12:03:08.602576075Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"\n"}
{"Time":"2020-03-28T12:03:08.602584429Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"goroutine 403 [running]:\n"}
{"Time":"2020-03-28T12:03:08.602590561Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"github.com/hashicorp/nomad/vendor/github.com/stretchr/testify/assert.Eventually.func1(0xc000a78000, 0xc0009cf160)\n"}
{"Time":"2020-03-28T12:03:08.602598464Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"\t/home/circleci/go/src/github.com/hashicorp/nomad/vendor/github.com/stretchr/testify/assert/assertions.go:1494 +0x47\n"}
{"Time":"2020-03-28T12:03:08.602604952Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"created by github.com/hashicorp/nomad/vendor/github.com/stretchr/testify/assert.Eventually\n"}
```
