This refactors precompile_cache.js to use the resource protocol handler to get the gre and app resource locations, and import all modules and components that can be found in either the corresponding omnijar or directory. This will allow, in bug 780561, to avoid a two-step packaging, and in bug 755724, to properly precompile cache for both browser and metro without too much hassle.

As a bonus, it also removes the import exceptions logs, which are confusing to many people looking at tinderbox logs.

Backed out for pymake bustage. https://hg.mozilla.org/integration/mozilla-inbound/rev/bf9c0d80b36e

Interdiff for a fixup to avoid the windows breakage. The problem was that MOZ_STARTUP_CACHE ended up with a path in the form e:/build/something, and NS_NewLocalFile barfs on these (because it expects absolute file names with backslashes).