Fix the typo in GenerateExplicitModuleMap.py where the modulePath is
mis-spelled. Use the module path from the dependency output json file
directly instead, which means the script can be used to generate
non-caching build module map as well since the module path actually
matters for those builds.
Improve caching related tests to make them faster and test the situation
closer to what actually happens during explicit module build. Most
noticable changes are:
* Avoid swift stdlib dependency during the tests to save time
* Use dependency scanner output to construct test cases
* Update old test cases that try to simulate caching from file system
inputs to using only CAS inputs.