This is an attempt to ease dependency management for external driver
plugins, by avoiding requiring them to compile ugorji/go generated
files.  Plugin developers reported some pain with the brittleness of
ugorji/go dependency in particular, specially when using go mod, the
default go mod manager in golang 1.13.

Context
--------

Nomad uses msgpack to persist and serialize internal structs, using
ugorji/go library.  As an optimization, we use ugorji/go code generation
to speedup process and aovid the relection-based slow path.

We commit these generated files in repository when we cut and tag the
release to ease reproducability and debugging old releases.  Thus,
downstream projects that depend on release tag, indirectly depends on
ugorji/go generated code.

Sadly, the generated code is brittle and specific to the version of
ugorji/go being used.  When go mod picks another version of ugorji/go
then nomad (go mod by default uses release according to semver),
d...