Build: Remove hardcoded reference to x-plugins in build (#21773)

* Build: Remove hardcoded reference to x-plugins in build

The gradle build currenlty allows extra plugins to be hooked into the
elasticsearch build by placing under an x-plugins directory as a sibling
of the elasticsearch checkout. This change converts this directory to
one called elasticsearch-extra. The subdirectories of
elasticsearch-extra become first level projects in gradle (while before
they would have been subprojects of `:x-plugins`. Additionally, this
allows major version checkouts to be associated with extra plugins. For
example, you could have elasticsearch checked out as
`elasticsearch-5.x`, and a sibling `elasticsearch-5.x-extra` directory.

TESTING.asciidoc

@@ -474,10 +474,10 @@ gradle run --debug-jvm
 == Building with extra plugins
 Additional plugins may be built alongside elasticsearch, where their
 dependency on elasticsearch will be substituted with the local elasticsearch
-build. To add your plugin, create a directory called x-plugins as a sibling
-of elasticsearch. Checkout your plugin underneath x-plugins and the build
-will automatically pick it up. You can verify the plugin is included as part
-of the build by checking the projects of the build.
+build. To add your plugin, create a directory called elasticsearch-extra as
+a sibling of elasticsearch. Checkout your plugin underneath elasticsearch-extra
+and the build will automatically pick it up. You can verify the plugin is
+included as part of the build by checking the projects of the build.
 
 ---------------------------------------------------------------------------
 gradle projects

settings.gradle

@@ -1,4 +1,5 @@
-rootProject.name = 'elasticsearch'
+String dirName = rootProject.projectDir.name
+rootProject.name = dirName
 
 List projects = [
   'build-tools',
@@ -86,7 +87,7 @@ if (isEclipse) {
 /**
   * Iterates over sub directories, looking for build.gradle, and adds a project if found
   * for that dir with the given path prefix. Note that this requires each level
-  * of the dir hiearchy to have a build.gradle. Otherwise we would have to iterate
+  * of the dir hierarchy to have a build.gradle. Otherwise we would have to iterate
   * all files/directories in the source tree to find all projects.
   */
 void addSubProjects(String path, File dir) {
@@ -96,17 +97,18 @@ void addSubProjects(String path, File dir) {
 
   String projectName = "${path}:${dir.name}"
   include projectName
+  if (path.isEmpty()) {
+    project(projectName).projectDir = dir
+  }
   for (File subdir : dir.listFiles()) {
     addSubProjects(projectName, subdir)
   }
 }
 
 // look for extra plugins for elasticsearch
-File xplugins = new File(rootProject.projectDir.parentFile, 'x-plugins')
-if (xplugins.exists()) {
-  include ':x-plugins'
-  project(':x-plugins').projectDir = xplugins
-  for (File extraPluginDir : xplugins.listFiles()) {
-    addSubProjects(':x-plugins', extraPluginDir)
+File extraProjects = new File(rootProject.projectDir.parentFile, "${dirName}-extra")
+if (extraProjects.exists()) {
+  for (File extraProjectDir : extraProjects.listFiles()) {
+    addSubProjects('', extraProjectDir)
   }
 }