WIP: fix JAR/JAD download regression

The normalize-dir-list branch refactored the *load* function, in the middle of which we landed a different *load* function in index.js, which caused a conflict, so bd0ff96 renames/refactors the new one. Not sure that's the ideal solution, but it should resolve that regression.

Unfortunately, it still doesn't resolve the issue I'm seeing of a persistent "download failure" on startup. According to my tests, the XHR in renamed *loadWithProgress* function has its *onerror* function called with *xhr.statusCode* `0` but *xhr.statusText* `OK`. Perhaps this is to do with SystemXHR?