Create the build command/plugin

Makefile

+NO_COLOR=\x1b[0m
+OK_COLOR=\x1b[32;01m
+ERROR_COLOR=\x1b[31;01m
+WARN_COLOR=\x1b[33;01m
+
 all:
 	@mkdir -p bin/
 	go get -d -v ./...
+	@echo "$(OK_COLOR)--> Compiling Packer...$(NO_COLOR)"
 	go build -v -o bin/packer
+	@echo "$(OK_COLOR)--> Compiling Command: Build...$(NO_COLOR)"
+	$(MAKE) -C plugin/command-build
 
 format:
 	go fmt ./...
 
 test:
+	@echo "$(OK_COLOR)--> Testing Packer...$(NO_COLOR)"
 	@go list -f '{{range .TestImports}}{{.}}\
 		{{end}}' ./... | xargs -n1 go get -d
 	go test ./...

command/build/command.go

-package main
+package build
 
 import "github.com/mitchellh/packer/packer"
 
-type buildCommand byte
+type Command byte
 
 func (Command) Run(env packer.Environment, arg []string) int {
 	env.Ui().Say("BUILDING!")

command/build/command_test.go

+package build
+
+

command/build/main.go

-package main
-
-import "github.com/mitchellh/packer/packer/plugin"
-
-func main() {
-	plugin.ServeCommand(new(buildCommand))
-}

plugin/command-build/Makefile

+plugin:
+	go get -d -v ./...
+	go build -v -o $(CURDIR)/bin/packer-build
+
+format:
+	go fmt ./...
+
+test:
+	@go list -f '{{range .TestImports}}{{.}}\
+		{{end}}' ./... | xargs -n1 go get -d
+	go test ./...
+
+.PHONY: all format test

plugin/command-build/main.go

+package main
+
+import (
+	"github.com/mitchellh/packer/command/build"
+	"github.com/mitchellh/packer/packer/plugin"
+)
+
+func main() {
+	plugin.ServeCommand(new(build.Command))
+}