[folder structure] Cleanup missing folder changes, update docs

docs/plugins.asciidoc

@@ -3,7 +3,7 @@
 
 Add-on functionality for Kibana is implemented with plug-in modules. You can use the `bin/kibana-plugin`
 command to manage these modules. You can also install a plugin manually by moving the plugin file to the
-`installedPlugins` directory and unpacking the plugin files into a new directory.
+`plugins` directory and unpacking the plugin files into a new directory.
 
 A list of existing Kibana plugins is available on https://github.com/elastic/kibana/wiki/Known-Plugins[GitHub].
 
@@ -63,7 +63,7 @@ Use the `remove` command to remove a plugin, including any configuration informa
 [source,shell]
 $ bin/kibana-plugin remove timelion
 
-You can also remove a plugin manually by deleting the plugin's subdirectory under the `installedPlugins/` directory.
+You can also remove a plugin manually by deleting the plugin's subdirectory under the `plugins/` directory.
 
 [float]
 === Listing Installed Plugins

src/cli/cluster/cluster_manager.js

@@ -87,7 +87,7 @@ module.exports = class ClusterManager {
 
     const watchPaths = uniq(
       [
-        fromRoot('src/plugins'),
+        fromRoot('src/core_plugins'),
         fromRoot('src/server'),
         fromRoot('src/ui'),
         fromRoot('src/utils'),

src/optimize/base_optimizer.js

@@ -114,7 +114,7 @@ class BaseOptimizer {
           { test: /\.(html|tmpl)$/, loader: 'raw' },
           { test: /\.png$/, loader: 'url?limit=10000&name=[path][name].[ext]' },
           { test: /\.(woff|woff2|ttf|eot|svg|ico)(\?|$)/, loader: 'file?name=[path][name].[ext]' },
-          { test: /[\/\\]src[\/\\](plugins|ui)[\/\\].+\.js$/, loader: `rjs-repack${mapQ}` },
+          { test: /[\/\\]src[\/\\](core_plugins|ui)[\/\\].+\.js$/, loader: `rjs-repack${mapQ}` },
           {
             test: /\.js$/,
             exclude: babelExclude.concat(this.env.noParse),