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.
The blake2b package only includes an asm implementation for amd64. Other
architectures use a generic Go implementation. Unfortunately appengine
is one of those architectures and we explicitly ignore appengine when
vendoring because it is an unsupported platform for Nomad.
Commit 925ec07a4 started ignoring appengine to help trim down the vendor
package. We now need to include appengine again - despite not building
for it - just to avoid accidently breaking other archs we do support.
