fix: Use default plugin if the path property is missing from a single plugin configuration

lib/plugins/definitions.js

@@ -1,17 +1,12 @@
 const {isString, isObject, isFunction, isArray} = require('lodash');
 const semver = require('semver');
-const conditionTravis = require('@semantic-release/condition-travis');
-const commitAnalyzer = require('@semantic-release/commit-analyzer');
-const releaseNotesGenerator = require('@semantic-release/release-notes-generator');
-const npm = require('@semantic-release/npm');
-const github = require('@semantic-release/github');
 
 const RELEASE_TYPE = ['major', 'premajor', 'minor', 'preminor', 'patch', 'prepatch', 'prerelease'];
 const validatePluginConfig = conf => isString(conf) || isString(conf.path) || isFunction(conf);
 
 module.exports = {
   verifyConditions: {
-    default: [npm.verifyConditions, github.verifyConditions, conditionTravis],
+    default: ['@semantic-release/npm', '@semantic-release/github', '@semantic-release/condition-travis'],
     config: {
       validator: conf => !conf || (isArray(conf) ? conf : [conf]).every(conf => validatePluginConfig(conf)),
       message:
@@ -19,7 +14,7 @@ module.exports = {
     },
   },
   getLastRelease: {
-    default: npm.getLastRelease,
+    default: '@semantic-release/npm',
     config: {
       validator: conf => Boolean(conf) && validatePluginConfig(conf),
       message:
@@ -35,7 +30,7 @@ module.exports = {
     },
   },
   analyzeCommits: {
-    default: commitAnalyzer,
+    default: '@semantic-release/commit-analyzer',
     config: {
       validator: conf => Boolean(conf) && validatePluginConfig(conf),
       message:
@@ -55,7 +50,7 @@ module.exports = {
     },
   },
   generateNotes: {
-    default: releaseNotesGenerator,
+    default: '@semantic-release/release-notes-generator',
     config: {
       validator: conf => !conf || validatePluginConfig(conf),
       message:
@@ -67,7 +62,7 @@ module.exports = {
     },
   },
   publish: {
-    default: [npm.publish, github.publish],
+    default: ['@semantic-release/npm', '@semantic-release/github'],
     config: {
       validator: conf => Boolean(conf) && (isArray(conf) ? conf : [conf]).every(conf => validatePluginConfig(conf)),
       message: