Selfupdate fix

This fixed 2 related issue of selfupdate:

1. *haxelib.json* was missing `classPath`, so `haxelib path haxelib_client` will output wrong path to the source of haxelib. And the final shell script produced by selfupdate would be with wrong `-cp`.

2. Selfupdate should overwrite the actual haxelib file instead of the symlink to it.