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Load RL modules in one load.php request, rather than in two stages

Authored by Catrope.
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Description

Load RL modules in one load.php request, rather than in two stages

This introduces TargetLoader, which manages plugins and RL modules
in a slightly more generic fashion so that Targets themselves don't
have to. This allows us to load all RL modules in one load.php
request, rather than first loading ViewPageTarget which then
loads the other modules.

TargetLoader loads in the bottom queue, so it will be loaded
as part of the main load.php request, but in VPT.init.js we
still have to wait for it with using() because it might not
have arrived yet. This also degrades gracefully on cached pages
where TargetLoader isn't in the bottom queue: it'll be loaded
as a separate request instead, which is suboptimal but no
worse that what we were doing before.

Right now TargetLoader is small enough that it could also be in
the top queue, but in the future we want to add things like
the action=visualeditor API request to it, and mw.Api is
relatively big.

Note: this also makes a breaking change to the plugin API:
plugin callbacks no longer receive the target instance
as a parameter, as they're now executed before the target
has been constructed rather than after. In the long term,
if we want to give plugins access to the target instance,
we could give them the target promise somehow. For now,
I've killed this feature because nothing used it and
the change from a direct object reference to a promise
would have been a breaking change anyway.

Also fixed incorrect documentation index for ve.init.mw.ViewPageTarget.init.

Bug: T53569
Change-Id: Ibfa6abbeaf872ae2aadc6ed9d5beba7473ea441a
(cherry picked from commit d371014e5dfe54d21986cff041bfb92e8e390a45)