We must keep track of the exact order in which modules are requested.
To do so, maintain a map from module specifiers to position while
parsing (in ModuleDescriptor). Descriptor entries now refer to that
position rather than the string.  When generating the ModuleInfo, turn
this map into an array of specifiers. We don't need the map anymore
later on, so we do not reconstruct it when deserializing again.

BUG=v8:1569

Review-Url: https://codereview.chromium.org/2353633002
Cr-Commit-Position: refs/heads/master@{#39519}