Add support for reading configs

This is the first step in attempting to read a Packer configuration file
via the packer init command. At the present moment it will try to read
one or more configuration templates from a given directory or path. Once
parsed it will error if parsing fails or exist successfully if it is
able to parse the file.

Looking at how the code is structured there will need to be changes made
to the following places:

- When no configuration file is found Packer will display an error. That
error should be bubbled up a bit so that the caller command can
determine if it should be displayed or not. For packer init no
configuration is not an error. Maybe it should be?

- After a configuration has been parsed there needs to be a single way
to determine a list of plugins associated with the configuration. HCL
and JSON configs have fields for this data but some is exported and some
is unexported. Adapting the packerHandler interface may be an option
here. More investigation needed.

command/cli.go

@@ -146,12 +146,14 @@ type HCL2UpgradeArgs struct {
 func (ia *InitArgs) AddFlagSets(flags *flag.FlagSet) {
 	flags.StringVar(&ia.PluginDir, "plugin-dir", "", "")
 	flags.BoolVar(&ia.GetPlugins, "get-plugins", true, "")
+	flags.BoolVar(&ia.Upgrade, "upgrade", false, "")
 	ia.MetaArgs.AddFlagSets(flags)
 }
 
 //InitArgs respresents a parsed cli line for packer init
 type InitArgs struct {
 	MetaArgs
-	PluginDir  string
 	GetPlugins bool
+	PluginDir  string
+	Upgrade    bool
 }

command/init.go

@@ -3,6 +3,7 @@ package command
 import (
 	"context"
 	"fmt"
+	"os"
 
 	"github.com/hashicorp/packer/version"
 )
@@ -33,17 +34,35 @@ func (c *InitCommand) ParseArgs(args []string) (*InitArgs, int) {
 		return &cfg, 1
 	}
 
+	args = flags.Args()
+	if len(args) > 0 {
+		cfg.Path = args[0]
+	}
+
+	// Init command should treat an empty invocation as if it was run
+	// against the current working directory.
+	if cfg.Path == "" {
+		cfg.Path, _ = os.Getwd()
+	}
+
 	return &cfg, 0
 }
 
 func (c *InitCommand) RunContext(ctx context.Context, cla *InitArgs) int {
-	fmt.Printf(`%s
+
+	packerStarter, ret := c.GetConfig(&cla.MetaArgs)
+	if ret != 0 {
+		fmt.Printf(`%s
 
 Packer initialized with no template!
 
 The directory has no Packer templates. You may begin working
 with Packer immediately by creating a Packer template.
 `, version.FormattedVersion())
+		return ret
+	}
+
+	_ = packerStarter.Initialize()
 
 	return 0
 }
@@ -64,8 +83,9 @@ Usage: packer init [options]
   provisioners, post-processors with changes in the template configuration file.
 
 Options:
-  -get-plugins=false                Skips plugin installation.
-  -plugin-dir=PATH                  Skips plugin installation and loads plugins only from the specified directory.
+  -get-plugins=false            Skips plugin installation.
+  -plugin-dir=PATH              Skips plugin installation and loads plugins only from the specified directory.
+  -upgrade=true                 Updates installed plugins to the latest available version.
 `
 	return helpText