rustdoc: make srcIndex no longer a global variable

[lolbinarycat]

this is one-time initialization data, it can just
be a function parameter.

while we're doing that, we can more the json parsing into the function and save a few extra bytes of storage for free, at least in the case of multiple crates in a doc bundle.

fixes #138467

[rustbot]

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[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha, @lolbinarycat

Some changes occurred in GUI tests.

cc @GuillaumeGomez

[GuillaumeGomez]

Seems good to me, thanks! Please remove the left-over debugging and then I'll approve.

[notriddle]

Why does this exist? searchIndex isn't part of the intended external interface, either.

[lolbinarycat]

I mean, at least the search index is something we actually want to persist, but I suppose it could just live in a closure upvar instead of a global.

The results table needs to be public for integration testing, although i'm not sure if the same is true of searchIndex (it's definitely not true for srcIndex, I checked)