Based on standard practices, we need to have a copyright and license notice at the top of each source file. Removed existing copyrights and updated/added copyright notices for .h, .cpp, .cs, .ps1, and .psm1 files.
Updated module manifests for consistency to have Author = "PowerShell" and Company = "Microsoft Corporation". Removed multiple line breaks.
Separate PR coming to update contribution document for new source files: #6140
Manually reviewed each change.
Fix#6073
Use 2.0 CLI/SDK of version `2.0.0-preview1-005952` to keep in sync with the .NET Core 2.0 //Build dogfood instructions. The version of `Microsoft.NetCore.App` in use is `2.0.0-preview1-002106-00`.
