Add a hook to StartUpModule that can modify the source URLs
that get embedded into the startup module JS in the form of
mw.loader.addSource() parameters.
The intended use case is preserving relevant URL information,
especially in the case of the 'local' source which is a
relative URL, when the URL pattern of the load.php URL
that generates the startup module code is different from the
URL pattern of the page that loads that load.php URL.
(E.g. $wgLoadScript is set to a different domain than the
current page, to canonicalize load.php URLs for a site
that's reachable via multiple domains, but then by default
the local source is a relative URL which would be
interpreted relative to the URL of the page that executes
the startup module code, so it would result in calls to
non-canonical load.php URLs).
Bug: T365162
Bug: T371530
Change-Id: I199ab779abd0596b836ae43dcc5f2f2a489c9274
31f614f732