The parent classloader of the Scala compiler. It was originally copied and pasted from https://github.com/lampepfl/dotty/blob/ea65338e06142f41f0e68b23250c8e22b0ba8837/sbt-dotty/src/dotty/tools/sbtplugin/DottyPlugin.scala#L586-L627

To understand why a custom parent classloader is needed for the compiler, let us describe some alternatives that wouldn't work.

- new URLClassLoader(urls): The compiler contains sbt phases that callback to sbt using the xsbti.* interfaces. If urls does not contain the sbt interfaces we'll get a ClassNotFoundException in the compiler when we try to use them, if urls does contain the interfaces we'll get a ClassCastException or a LinkageError because if the same class is loaded by two different classloaders, they are considered distinct by the JVM.

- new URLClassLoader(urls, bloopLoader): Because of the JVM delegation model, this means that we will only load a class from urls if it's not present in the parent bloopLoader, but Bloop uses its own version of the scala library which is not the one we need to run the compiler.

Our solution is to implement an URLClassLoader whose parent is new ScalaInstanceTopLoader(bloopClassLoader, bootClassLoader). We override loadClass to load the xsbti.* interfaces from the bloopClassLoader and the JDK classes from the bootClassLoader.