resourceloader: Clean up and better document module list (un)packing

resourceloader: Clean up and better document module list (un)packing

* Move buildModulesString() call from doRequest() to batchRequest()

  This keeps all module string "packing" logic located to the same function,
  which is batchRequest(). It also means that the moduleMap object will not
  leave the function, which helps in maintenance given it's very internal.

* Add comments to all the methods referring to each other.

* Explain why buildModulesString() is only a partial port, and the rest is
  inlined in batchRequest().

* Minor changes to the JS and PHP implementation to better match each other.
  - '$groups' -> '$moduleMap'.
  - Remove redundant '$str'.

Bug: T188076
Change-Id: I7b0790606c456e492ab682faeb80f7e7fce7d9f8