resourceloader: Implement support for 'site' into mw.loader
- No longer a dedicated <script> with only=scripts. This means it creates no extra script request and becomes a versioned request using data from the startup module.
- No longer in group=site. This means it collapses into the existing bottom queue. Not even one dedicated script request, but zero.
- No longer exclude from module storage. This can be cached like any other module. It was previously excluded because it was already loaded separately.
- Change mw.loader#execute to special-case the 'site' module with $.globalEval.