resourceloader: Implement modern module loading (1/2)

resourceloader: Implement modern module loading (1/2)

This defines mw.loader.require() and 'module.exports'. These will
be exposed to mw.loader.implement() closures as local 'require'
and 'module' parameters.

Changes:
* This alters nestedAddScript to maintain a single queue to
  ensure scripts from different modules are never downloaded in
  parallel (used in debug mode).

Note:
A further patch will start passing module and require to module definitions.

Bug: T108655
Change-Id: Ia925844cc22f143f531216f2fe3efead08885b5d