plugin: factorise calling describe on binaries

Given that calling the describe command on plugins and deserialising the
output as a plugin description is something done in multiple places in
the code, we factorise this operation so we don't need to copy/paste the
code around.

packer/plugin-getter/plugins.go

@@ -211,18 +211,12 @@ func (pr Requirement) ListInstallations(opts ListInstallationsOptions) (InstallL
 			}
 		}
 
-		descOut, err := exec.Command(path, "describe").Output()
+		describeInfo, err := GetPluginDescription(path)
 		if err != nil {
-			log.Printf("couldn't call describe on %q, ignoring", path)
+			log.Printf("failed to call describe on %q: %s", path, err)
 			continue
 		}
 
-		var describeInfo pluginsdk.SetDescription
-		err = json.Unmarshal(descOut, &describeInfo)
-		if err != nil {
-			log.Printf("%q: describe output deserialization error %q, ignoring", path, err)
-		}
-
 		// versionsStr now looks like v1.2.3_x5.1 or amazon_v1.2.3_x5.1
 		parts := strings.SplitN(versionsStr, "_", 2)
 		pluginVersionStr, protocolVersionStr := parts[0], parts[1]
@@ -933,6 +927,18 @@ func (pr *Requirement) InstallLatest(opts InstallOptions) (*Installation, error)
 	return nil, errs
 }
 
+func GetPluginDescription(pluginPath string) (pluginsdk.SetDescription, error) {
+	out, err := exec.Command(pluginPath, "describe").Output()
+	if err != nil {
+		return pluginsdk.SetDescription{}, err
+	}
+
+	desc := pluginsdk.SetDescription{}
+	err = json.Unmarshal(out, &desc)
+
+	return desc, err
+}
+
 func init() {
 	var err error
 	// Should never error if both components are set

packer/plugin.go

@@ -5,7 +5,6 @@ package packer
 
 import (
 	"crypto/sha256"
-	"encoding/json"
 	"fmt"
 	"log"
 	"os"
@@ -127,13 +126,9 @@ func (c *PluginConfig) Discover() error {
 // if the "packer-plugin-amazon" binary had an "ebs" builder one could use
 // the "amazon-ebs" builder.
 func (c *PluginConfig) DiscoverMultiPlugin(pluginName, pluginPath string) error {
-	out, err := exec.Command(pluginPath, "describe").Output()
+	desc, err := plugingetter.GetPluginDescription(pluginPath)
 	if err != nil {
-		return err
-	}
-	var desc pluginsdk.SetDescription
-	if err := json.Unmarshal(out, &desc); err != nil {
-		return err
+		return fmt.Errorf("failed to get plugin description from executable %q: %s", pluginPath, err)
 	}
 
 	pluginPrefix := pluginName + "-"