Refactor the code base to make it easy to support concurrency within a worker in future

[daxian-dbw]

Refactor the code base to make it easy to support concurrency within a worker in future.

• Add PowerShellManagerPool , but it's just the skeleton. Today the pool only has one PowerShellManager instance. We can add the real pool implementation if we decide to support concurrency within a worker process.
• Setup the PSModule environment variable as part of Runspace.Open by using InitialSessionState.EnvironmentVariables. This fixes the mysterious ModulePathShouldBeSetCorrectly test failure by making sure the env variable is set to the expected one as part of the Runspace.Open of every PowerShellManager. It failed before because:
  1. dotnet test runs tests in parallel by default
  2. when creating a new PowerShellManager, the PSModulePath environment variable will be set again within Runspace.Open (by SetModulePath called from ModuleIntrisic constructor). That makes value unexpected.
• Update tests to make them more reliable.

[TylerLeonhardt]

This looks great Dongbo 😃

I just had a couple questions