[IMPR] Create a SiteLink with getitem method
SiteLinkCollection initializer updates data contents to self._data
dict. The update method calls setitem for each data entry; for
each entry a SiteLink is created which needs a siteinfo content for
every affected site and leads to a huge loading time.
Now with this patch the SiteLink object is created when the link
is fetched from self._data via getitem or iter method.
This prevents siteinfo content for each site from loading even
a link is currently not used and decrease loading time a lot.