Lock bundled modules (especially PSReadLine) #1493

Per PowerShell/vscode-powershell#3385 users can encounter incompatibility issues by installing newer versions of our dependencies since we do not lock PSES to the bundled version. This is unfortunate and causes more issues than we believe are fixed by picking up the latest installed version. We are going to change PSES to only load the bundled module so that we, the developers, can manually update and test our dependencies. In dire scenarios, users can still just overwrite the bundled module in the extension folder, and at that point will either be following our specific instructions for a workaround or going outside our supported scenarios.