optimize d2x pkgindex repo, avoid install dir issue for new xlings version (auto .stamp)

Author: Sunrisepeak

template.lua

@@ -28,9 +28,18 @@ function installed()
 end
 
 function install()
-    local dest = d2x_projectdir()
-    os.cp(projectdir, dest)
+    os.cp(projectdir, pkginfo.install_dir())
     os.tryrm(projectdir)
+    return true
+end
+
+function config()
+    local dest = d2x_projectdir()
+    if os.isdir(dest) then
+        log.warn("Course '%s' is already installed at '%s', skipping config", package.name, dest)
+        return true
+    end
+    os.cp(pkginfo.install_dir(), dest)
     os.cd(dest)
     system.exec("xlings install")
     return true