Simplify ClassLoader isolation in launch command

No more SBT-inspired ClasspathFilter. It now just tries to find the ClassLoader that loaded coursier, and use the parent as a base to load the launched application.

Allows to remove the -b option of the bootstrap command, and remove BaseCommand (that was holding options common to all commands).

bootstrap/src/main/java/coursier/Bootstrap.java

@@ -8,7 +8,6 @@ import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.net.URI;
 import java.net.URL;
-import java.net.URLClassLoader;
 import java.net.URLConnection;
 import java.nio.file.Files;
 import java.security.CodeSource;
@@ -170,8 +169,6 @@ public class Bootstrap {
         String mainClass0 = System.getProperty("bootstrap.mainClass");
         String jarDir0 = System.getProperty("bootstrap.jarDir");
 
-        boolean prependClasspath = Boolean.parseBoolean(System.getProperty("bootstrap.prependClasspath", "false"));
-
         final File jarDir = new File(jarDir0);
 
         if (jarDir.exists()) {
@@ -267,7 +264,7 @@ public class Bootstrap {
             parentClassLoader = new IsolatedClassLoader(contextURLs, parentClassLoader, new String[]{ isolationID });
         }
 
-        URLClassLoader classLoader = new URLClassLoader(localURLs.toArray(new URL[localURLs.size()]), parentClassLoader);
+        ClassLoader classLoader = new BootstrapClassLoader(localURLs.toArray(new URL[localURLs.size()]), parentClassLoader);
 
         Class<?> mainClass = null;
         Method mainMethod = null;
@@ -288,14 +285,6 @@ public class Bootstrap {
 
         List<String> userArgs0 = new ArrayList<>();
 
-        if (prependClasspath) {
-            for (URL url : localURLs) {
-                assert url.getProtocol().equals("file");
-                userArgs0.add("-B");
-                userArgs0.add(url.getPath());
-            }
-        }
-
         for (int i = 0; i < args.length; i++)
             userArgs0.add(args[i]);
 

bootstrap/src/main/java/coursier/BootstrapClassLoader.java

@@ -0,0 +1,25 @@
+package coursier;
+
+import java.net.URL;
+import java.net.URLClassLoader;
+
+public class BootstrapClassLoader extends URLClassLoader {
+
+    public BootstrapClassLoader(
+            URL[] urls,
+            ClassLoader parent
+    ) {
+        super(urls, parent);
+    }
+
+    /**
+     * Can be called by reflection by launched applications, to find the "main" `ClassLoader`
+     * that loaded them, and possibly short-circuit it to load other things for example.
+     *
+     * The `launch` command of coursier does that.
+     */
+    public boolean isBootstrapLoader() {
+        return true;
+    }
+
+}