This reverts commit 3e6f737f.

Previously, it was required that you `go get github.com/hashicorp/nomad` to be
able to build protos, as the protoc invocation added an include directive that
pointed to `$GOPATH/src`, which is how dependent protos were discovered. As
Nomad now uses Go modules, it won't necessarily be cloned to `$GOPATH`.
(Additionally, if you _had_ go-gotten Nomad at some point, protoc compilation
would have possibly used the _wrong_ protos, as those wouldn't necessarily be
the most up-to-date ones.)

This change modifies the proto files and the `protoc` invocation to handle
discovering dependent protos via protoc plugin modifier statements that are
specific to the protoc plugin being used.

In this change, `make proto` was run to recompile the protos, which results in
changes only to the gzipped `FileDescriptorProto`.

client/fingerprint: detect unloaded dynamic bridge kernel module

HCL2: Variables and Locals

add CHANGELOG entry for #9296

Remove Namespace field from JobScaleRequest

This reimplements the handling of Variables and Locals in HCL2 config.

This change supports declaring variables and locals, with defaults.

In Nomad v0.12.0, the client added additional fingerprinting around the
presense of the bridge kernel module. The fingerprinter only checked in
`/proc/modules` which is a list of loaded modules. In some cases, the
bridge kernel module is builtin rather than dynamically loaded. The fix
for that case is in #8721. However we were still missing the case where
the bridge module is dynamically loaded, but not yet loaded during the
startup of the Nomad agent. In this case the fingerprinter would believe
the bridge module was unavailable when really it gets loaded on demand.

This PR now has the fingerprinter scan the kernel module dependency file,
which will contain an entry for the bridge module even if it is not yet
loaded.

In summary, the client now looks for the bridge kernel module in
 - /proc/modules
 - /lib/modules/<kernel>/modules.builtin
 - /lib/modules/<kernel>/modules.dep

Closes #8423

Small changes to the Windows 2016 Packer build for debuggability of
provisioning:

* improve verbosity of powershell error handling
* remove unused "tools" installation
* use ssh communicator for Packer to improve Packer build times and eliminate
  deprecated winrm remote access (unavailable from current macOS)

* append custom path to custom_config_files

* remove config_path variable