command: plugins install reject non-releases

When installing a plugin with packer plugins install --path, we only
accept release versions of a plugin, as otherwise the loading can be
inconsistent if for example a user specifies a required_plugins block in
their template, in which case the plugins will be ignored.

Until we have a simpler loading scheme then, we will reject non-release
versions of plugins to avoid confusion.

command/plugins_install.go

@@ -263,6 +263,14 @@ an issue on our Github repo to signal it.`)
 		}})
 	}
 
+	if strings.Contains(desc.Version, "-") {
+		return writeDiags(c.Ui, nil, hcl.Diagnostics{&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Invalid version",
+			Detail:   fmt.Sprintf("Packer can only install plugin releases with this command (ex: 1.0.0), the binary's reported version is %q", desc.Version),
+		}})
+	}
+
 	pluginBinary, err := os.Open(args.PluginPath)
 	if err != nil {
 		return writeDiags(c.Ui, nil, hcl.Diagnostics{&hcl.Diagnostic{