Improve script fetching algorithm clarity

This is almost all editorial, but it fixes one bug that was referring to the no-longer-existing "top-level module fetch" flag, when it should instead be referring to the "isTopLevel" variable introduced in f3550cd.

The editorial changes are:

* Adding types to all the parameters of the script fetching algorithms.
* Converting variables and parameters in those algorithms to camelCase.

Fixes #7996.