resourceloader: Preload base modules request from startup module

resourceloader: Preload base modules request from startup module

This allows browsers that support W3C Preload <https://www.w3.org/TR/preload/>,
to start downloading and parsing the second load.php JavaScript request
as soon as the headers of the startup module arrive.

Before:
- HTML request, response start.
|\- Discover <script src async> for startup.js, request, response start.
| \ - Startup response end, JS parsing.
|  |
|__|- HTML parsing and rendering yields for async JS execution.
   \- JavaScript inserts <script>, Base-modules request starts. <<<

After:
- HTML request, response start.
|\- Discover <script src async> for startup.js, request, response start
  \ - Base-modules request starts. <<<
|  |- Startup response end, JS parsing.
|  |
|__|- HTML parsing and rendering yields for async JS execution.
   \- JavaScript inserts <script>. (Base-modules req on-going, or done)

On local testing with "Slow 3G" network throttling in Chrome,
this reduced page load time (window.onload, time to interaction for JS)
by about 1 second (from 14s to 13s).
See also https://phabricator.wikimedia.org/T164299#3572231

Bug: T164299
Change-Id: I7047f4ab881947cf3392256087cc5a0cb177dd3a