Banish build-eclipse (elastic#70696)

Move it to `out` so we can be like the cool kids.

.gitignore

@@ -5,6 +5,7 @@
 *.ipr
 *.iws
 build-idea/
+# Eclipse and Intellij put there build files in "out"
 out/
 
 # include shared intellij config
@@ -19,6 +20,7 @@ benchmarks/src/main/generated/*
 .project
 .classpath
 .settings
+# We don't use this any more, but we'll keep it around in gitignore for a while so we don't accidentally commit it
 build-eclipse/
 
 # netbeans files

build.gradle

@@ -343,13 +343,17 @@ allprojects {
   }
 
   plugins.withType(JavaBasePlugin) {
-    eclipse.classpath.defaultOutputDir = file('build-eclipse')
     eclipse.classpath.file.whenMerged { classpath ->
-      // give each source folder a unique corresponding output folder
+      /*
+       * give each source folder a unique corresponding output folder
+       * outside of the usual `build` folder. We can't put the build
+       * in the usual build folder because eclipse becomes *very* sad
+       * if we delete it. Which `gradlew clean` does all the time.
+       */
       int i = 0;
       classpath.entries.findAll { it instanceof SourceFolder }.each { folder ->
         i++;
-        folder.output = "build-eclipse/" + i
+        folder.output = 'out/eclipse/' + i
       }
 
       // Starting with Gradle 6.7 test dependencies are not exposed by eclipse