Further changes/improvement needed to module path calculation #1744

While working on the sxs module path, I found some method names are intuitive, for example, GetDscModulePath() actually returns the Program Files shared module path, GetSystemWideModulePath() actually returns the pshome module path. Those method need to be renamed.

• Refactor method names related to module path calculation
• The code that detects if a module path is PSHome module path derived from parent process needs to be smarter, not only check powershell file in the parent folder, but maybe check powershell.deps.json file as well, and also the short folder name of the path should be Modules. The current detection logic casued a test to fail in Travis CI, see Fix the test that fails in Travis CI build #1792
• Add more SxS module path tests
• Think about the name inbox of Platform.IsInbox property. Do we need a better name? Platform.IsInbox is an internal property, and it's well commented to clarify its meaning. We will keep this name unless there is a big concern about it.