protos: Update .proto files not to use Go package name (#9301)

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`.

GNUmakefile

@@ -240,11 +240,21 @@ generate-structs: ## Update generated code
 	@echo "--> Running go generate..."
 	@go generate $(LOCAL_PACKAGES)
 
+## The ",M<path/to/proto>=<package name>" below is required to tell
+## protoc-gen-go what the import path for a given proto file should be. This is
+## necessary when a proto file foo/a.proto imports a proto file from another
+## directory, e.g. bar/b.proto
 .PHONY: proto
 proto:
 	@echo "--> Generating proto bindings..."
 	@for file in $$(git ls-files "*.proto" | grep -E -v -- "vendor\/.*.proto|demo\/.*.proto"); do \
-		protoc -I . -I $(shell go env GOPATH)/src --go_out=plugins=grpc:. $$file; \
+		protoc -I . --go_out=plugins=grpc\
+	,Mplugins/shared/hclspec/hcl_spec.proto=github.com/hashicorp/nomad/plugins/shared/hclspec\
+	,Mplugins/shared/structs/proto/stats.proto=github.com/hashicorp/nomad/plugins/shared/structs/proto\
+	,Mplugins/shared/structs/proto/attribute.proto=github.com/hashicorp/nomad/plugins/shared/structs/proto\
+	,Mplugins/shared/structs/proto/recoverable_error.proto=github.com/hashicorp/nomad/plugins/shared/structs/proto\
+	,Mplugins/drivers/proto/driver.proto=github.com/hashicorp/nomad/plugins/drivers/proto\
+	:. $$file; \
 	done
 
 .PHONY: generate-examples

drivers/shared/executor/proto/executor.proto

@@ -3,7 +3,7 @@ package hashicorp.nomad.plugins.executor.proto;
 option go_package = "proto";
 
 import "google/protobuf/timestamp.proto";
-import "github.com/hashicorp/nomad/plugins/drivers/proto/driver.proto";
+import "plugins/drivers/proto/driver.proto";
 
 service Executor {
     rpc Launch(LaunchRequest) returns (LaunchResponse) {}