Prevent copying folders inside themselves

- contains now performs a case-insensitive comparison. While this is a
   little dubious, it's required for systems like Windows, where foo and
   Foo are the same folder.
 - Impose a depth limit on copyRecursive. If there are any other cases
   where we may try to copy a folder into itself, this should prevent
   the computer entirely crashing.

src/test/java/dan200/computercraft/core/computer/BasicEnvironment.java

@@ -116,7 +116,7 @@ public class BasicEnvironment implements IComputerEnvironment
             while( baseFile != null && !wholeFile.exists() )
             {
                 baseFile = baseFile.getParentFile();
-                wholeFile = new File( baseFile, "resources/" + fallback + "/" + path );
+                wholeFile = new File( baseFile, "src/" + fallback + "/resources/" + path );
             }
 
             if( !wholeFile.exists() ) throw new IllegalStateException( "Cannot find ROM mount at " + file );