[java] cleanup: get rid of fake packages in content modules of Java Frontend plugin (IDEA-366600)

Fake packages were used to ensure that the main plugin classloader is used to load the classes from content modules. Now the same can be achieved explicitly with loading="embedded" attribute. This is also needed to ensure that the classpath for content modules can be properly specified (this is needed for IJPL-165692).

Also, it's possible to use auto-layout for the plugin, so there is no need to specify it explicitly.

GitOrigin-RevId: 6a75b4868b796215018d230949752bc978c60476

java/java-frontback-psi-impl/resources/intellij.java.frontback.psi.impl.xml

@@ -1,4 +1,4 @@
-<idea-plugin package="com.intellij.java.frontback.psi.impl.fake">
+<idea-plugin>
   <module value="com.intellij.java.frontback.psi.impl" />
   <extensions defaultExtensionNs="com.intellij">
     <!--suppress PluginXmlValidity -->

java/java-frontback-psi-impl/src/com/intellij/java/frontback/psi/impl/fake/placeholder.txt

@@ -1,2 +0,0 @@
-This file is added as a temporary workaround to have fake root package in intellij.java.frontback.psi.impl module.
-It can be removed when Java Frontend plugin is properly migrated to Plugin Model v2.