Use hashicorp/go-plugin for plugin system

This replaces this plugin system with the extracted hashicorp/go-plugin
library.

This doesn't introduce any new features such as binary flattening but
opens us up to that a lot more easily and removes a lot of code from TF
in favor of the upstream lib.

This will introduce a protocol change that will cause all existing
plugins to have to be recompiled to work properly. There is no actual
API changes so they just have to recompile, but it is technically
backwards incompatible.

config.go

@@ -9,8 +9,9 @@ import (
 	"path/filepath"
 	"strings"
 
+	"github.com/hashicorp/go-plugin"
 	"github.com/hashicorp/hcl"
-	"github.com/hashicorp/terraform/plugin"
+	tfplugin "github.com/hashicorp/terraform/plugin"
 	"github.com/hashicorp/terraform/terraform"
 	"github.com/kardianos/osext"
 )
@@ -202,7 +203,9 @@ func (c *Config) providerFactory(path string) terraform.ResourceProviderFactory
 	// Build the plugin client configuration and init the plugin
 	var config plugin.ClientConfig
 	config.Cmd = pluginCmd(path)
+	config.HandshakeConfig = tfplugin.Handshake
 	config.Managed = true
+	config.Plugins = tfplugin.PluginMap
 	client := plugin.NewClient(&config)
 
 	return func() (terraform.ResourceProvider, error) {
@@ -213,7 +216,12 @@ func (c *Config) providerFactory(path string) terraform.ResourceProviderFactory
 			return nil, err
 		}
 
-		return rpcClient.ResourceProvider()
+		raw, err := rpcClient.Dispense(tfplugin.ProviderPluginName)
+		if err != nil {
+			return nil, err
+		}
+
+		return raw.(terraform.ResourceProvider), nil
 	}
 }
 
@@ -232,8 +240,10 @@ func (c *Config) ProvisionerFactories() map[string]terraform.ResourceProvisioner
 func (c *Config) provisionerFactory(path string) terraform.ResourceProvisionerFactory {
 	// Build the plugin client configuration and init the plugin
 	var config plugin.ClientConfig
+	config.HandshakeConfig = tfplugin.Handshake
 	config.Cmd = pluginCmd(path)
 	config.Managed = true
+	config.Plugins = tfplugin.PluginMap
 	client := plugin.NewClient(&config)
 
 	return func() (terraform.ResourceProvisioner, error) {
@@ -242,7 +252,12 @@ func (c *Config) provisionerFactory(path string) terraform.ResourceProvisionerFa
 			return nil, err
 		}
 
-		return rpcClient.ResourceProvisioner()
+		raw, err := rpcClient.Dispense(tfplugin.ProvisionerPluginName)
+		if err != nil {
+			return nil, err
+		}
+
+		return raw.(terraform.ResourceProvisioner), nil
 	}
 }