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.

• Add hack to ensure the styles of the 'site' module still load without JavaScript, in the top, and after the ResourceLoaderDynamicStyles marker. This unfortunately stays its own request. Not sure how to avoid that.

Bug: T32358
Bug: T106736
Bug: T102077
Change-Id: I291a8c3aae1a71760bec58161891c1bd77c9b724