Use a standard gradle project structure.

I'm going to add tests next, but with this weird file structure I've been using, there's no good place to put them.

The only content changes here are in build.gradle.

PiperOrigin-RevId: 512914783

Author: plumpy

idea_plugin/build.gradle

@@ -50,13 +50,6 @@ publishPlugin {
   token = project.ext.properties.jetbrainsPluginRepoToken
 }
 
-sourceSets {
-  main {
-    java.srcDir "src"
-    resources.srcDir "resources"
-  }
-}
-
 dependencies {
   implementation "com.google.googlejavaformat:google-java-format:${googleJavaFormatVersion}"
 }