feat: support sharable configuration

Adds the options `extends`, which can be defined via configuration file or CLI arguments to a single path or an array of paths of shareable configuration.
A shareable configuration is a file or a module that can be loaded with `require`.
Options is defined by merging in the following order of priority:
- CLI/API
- Configuration file
- Shareable configuration (from right to left)

Options set in a shareable configuration can be unset by setting it to `null` or `undefined` in the main configuration file. If a default value applies to this property it will be used.

lib/plugins/normalize.js

@@ -1,17 +1,29 @@
+const {dirname} = require('path');
 const {inspect} = require('util');
+const SemanticReleaseError = require('@semantic-release/error');
 const {isString, isObject, isFunction, noop, cloneDeep} = require('lodash');
-const importFrom = require('import-from');
+const resolveFrom = require('resolve-from');
 
-module.exports = (pluginType, globalOpts, pluginOpts, logger, validator) => {
+module.exports = (pluginType, pluginsPath, globalOpts, pluginOpts, logger, validator) => {
   if (!pluginOpts) {
     return noop;
   }
+
   const {path, ...config} = isString(pluginOpts) || isFunction(pluginOpts) ? {path: pluginOpts} : pluginOpts;
   if (!isFunction(pluginOpts)) {
-    logger.log('Load plugin %s from %s', pluginType, path);
+    if (pluginsPath[path]) {
+      logger.log('Load plugin %s from %s in shareable config %s', pluginType, path, pluginsPath[path]);
+    } else {
+      logger.log('Load plugin %s from %s', pluginType, path);
+    }
   }
 
-  const plugin = isFunction(path) ? path : importFrom.silent(__dirname, path) || importFrom(process.cwd(), path);
+  const basePath = pluginsPath[path]
+    ? dirname(resolveFrom.silent(__dirname, pluginsPath[path]) || resolveFrom(process.cwd(), pluginsPath[path]))
+    : __dirname;
+  const plugin = isFunction(path)
+    ? path
+    : require(resolveFrom.silent(basePath, path) || resolveFrom(process.cwd(), path));
 
   let func;
   if (isFunction(plugin)) {
@@ -19,8 +31,9 @@ module.exports = (pluginType, globalOpts, pluginOpts, logger, validator) => {
   } else if (isObject(plugin) && plugin[pluginType] && isFunction(plugin[pluginType])) {
     func = plugin[pluginType].bind(null, cloneDeep({...globalOpts, ...config}));
   } else {
-    throw new Error(
-      `The ${pluginType} plugin must be a function, or an object with a function in the property ${pluginType}.`
+    throw new SemanticReleaseError(
+      `The ${pluginType} plugin must be a function, or an object with a function in the property ${pluginType}.`,
+      'EPLUGINCONF'
     );
   }