Add compiler-interface to the Scala Instance top loader

[adpi2]

The Scala 3 compiler now has a direct dependency on the org.scala-sbt:compiler-interface artifact. When compiling, it is crucial that its loads the compiler-interface classes from the same loader than sbt. Otherwise we have LinkageException.

In this PR I moved the compiler-interface up in the class loader hierarchy. It is safe because it's a pure java API that has no dependency. Rather than introducing a new loader in the pile of class loaders, I reused the TestInterfaceClassLoader and I renamed it SbtInterfaceClassLoader.

The hierarchy of class loader now looks like this (the parents are at the bottom):

• MetaBuildLoader: all sbt jars
• FullScalaLoader: scala-xml_2.12, scala-compiler-2.12.12 and scala-reflect-2.12.12
• LibraryLoader: scala-library-2.12.12
• JLineLoader: jline jars
• SbtInterfaceLoader: test-interface and compiler-interface
• topLoader: JDK except Jansi

Depending on useScalaReplJLine the scalaInstanceTopLoader is JLineLoader (which contains SbtInterfaceLoader) or SbtInterfaceLoader directly.

I tested it successfully against the 3.0.0-M2 compiler and a snapshot of the future 3.0.0-M3 compiler.

[eatkins]

LGTM