improve loadSlice to work like require_once

[pepone]

loadSlice in Ruby and Python parses a file and the generate code that is only executed if the definitions currently not exits

if 'S' not in _M_Test.__dict__:
    _M_Test.S = Ice.createTempClass()
    class S(object):

This works, but I think will be more correct that the extensions keeps track of the list of Slice files already loaded (canonical path), and a second load of the same file will do nothing.

With current code loading two different Slice files that happen to define the same type, the first one will be defined and the second silently ignored.

[pepone]

Python fixed in https://github.com/zeroc-ice/ice/commit/e2afab0e4b2da98288b23aa504fb56d53390b35e

[pepone]

Doing a file path check doesn't work with the --all option, which parses and generates code for all files simultaneously.