Add a module computer class which can compute modules for a package (#1574)

* add `_ModuleComputer` class and top level `computeModules` function.
  * uses tarjans algorithm to create strongly connected components first
  * then groups modules based on the entry points that import them
* move InMemoryModuleConfigManager to util.dart
* add equalsModule as a proper matcher, and makeAssets test utility
* add isPart function to lib/src/dart.dart and add proper support for part files