Hide Zinc Compilation implementation

Before, we had made this public in `sbt.inc`, but this is not necessary
anymore.

internal/zinc-ivy-integration/src/main/java/sbt/inc/ZincBridgeProvider.java

@@ -15,7 +15,7 @@ import xsbti.compile.CompilerBridgeProvider;
 import java.io.File;
 import java.util.concurrent.Callable;
 
-public final class ZincBridgeProvider {
+public interface ZincBridgeProvider {
     /**
      * Returns an ivy resolver to resolve dependencies locally in the default `.ivy2/local`.
      * <p>