src-script.js: make srcIndex into a parameter instead of a global variable

[lolbinarycat]

as far as I can tell, this is only used in createSrcSidebar, which is called in generated code like this:

var srcIndex = new Map(JSON.parse('[["test123",["",[],["test123.rs"]]]]'));
createSrcSidebar();

instead, we could have generated code like this:

let srcIndex = new Map(JSON.parse('[["test123",["",[],["test123.rs"]]]]'));
createSrcSidebar(srcIndex);

[lolbinarycat]

Working on this now. This should slightly reduce rustdoc's idle memory usage because we won't have a global variable floating around holding data used for one-time initialization.