Add system property for closing classloaders

I realized that some builds may crash if we automatically close the
classloaders. While I do think that is a good thing in general that we
are closing the loaders by default, we shuold have an option for
supressing this behavior.

I made all of the custom classloaders that we define for test and run
check this property before calling the super.close method.

main/src/main/java/sbt/internal/FlatLoader.java

@@ -7,6 +7,7 @@
 
 package sbt.internal;
 
+import java.io.IOException;
 import java.net.URL;
 import java.net.URLClassLoader;
 
@@ -29,4 +30,9 @@ final class FlatLoader extends URLClassLoader {
     }
     return "FlatLoader(\n  parent = " + getParent() + "\n  jars = " + jars.toString() + ")";
   }
+
+  @Override
+  public void close() throws IOException {
+    if (SysProp.closeClassLoaders()) super.close();
+  }
 }

main/src/main/java/sbt/internal/ScalaLibraryClassLoader.java

@@ -7,6 +7,7 @@
 
 package sbt.internal;
 
+import java.io.IOException;
 import java.net.URL;
 import java.net.URLClassLoader;
 
@@ -31,4 +32,9 @@ final class ScalaLibraryClassLoader extends URLClassLoader {
     }
     return "ScalaLibraryClassLoader(" + builder + " parent = " + getParent() + ")";
   }
+
+  @Override
+  public void close() throws IOException {
+    if (SysProp.closeClassLoaders()) super.close();
+  }
 }

main/src/main/java/sbt/internal/ScalaReflectClassLoader.java

@@ -7,6 +7,7 @@
 
 package sbt.internal;
 
+import java.io.IOException;
 import java.net.URL;
 import java.net.URLClassLoader;
 
@@ -26,4 +27,9 @@ final class ScalaReflectClassLoader extends URLClassLoader {
   public String toString() {
     return "ScalaReflectClassLoader(" + jar + " parent = " + getParent() + ")";
   }
+
+  @Override
+  public void close() throws IOException {
+    if (SysProp.closeClassLoaders()) super.close();
+  }
 }

main/src/main/scala/sbt/internal/LayeredClassLoaders.scala

@@ -31,6 +31,7 @@ private[internal] class LayeredClassLoaderImpl(
 ) extends URLClassLoader(classpath.map(_.toURI.toURL).toArray, parent)
     with NativeLoader {
   setTempDir(tempDir)
+  override def close(): Unit = if (SysProp.closeClassLoaders) super.close()
 }
 
 /**
@@ -146,6 +147,7 @@ private[internal] final class ReverseLookupClassLoaderHolder(
     }
     override def loadClass(name: String, resolve: Boolean): Class[_] =
       loadClass(name, resolve, reverseLookup = true)
+    override def close(): Unit = if (SysProp.closeClassLoaders) super.close()
   }
 
   /**
@@ -193,7 +195,7 @@ private[internal] final class ReverseLookupClassLoaderHolder(
     }
     override def close(): Unit = {
       checkin(parent)
-      super.close()
+      if (SysProp.closeClassLoaders) super.close()
     }
   }
 }

main/src/main/scala/sbt/internal/SysProp.scala

@@ -82,6 +82,8 @@ object SysProp {
    */
   lazy val color: Boolean = ConsoleAppender.formatEnabledInEnv
 
+  def closeClassLoaders: Boolean = getOrTrue("sbt.classloader.close")
+
   def supershell: Boolean = color && getOrTrue("sbt.supershell")
 
   def supershellSleep: Long = long("sbt.supershell.sleep", 100L)