Split Microsoft.VisualBasic.dll into nuget packages

Now, that `Microsoft.VisualBasic.dll` is about to be ported to .NET Core, it would be possible to untie this legacy code from the language itself and let it become nuget packages.

This has already been suggested in https://github.com/dotnet/vblang/issues/448#issuecomment-501920823 and https://github.com/dotnet/vblang/issues/448#issuecomment-504795301

## Details
- Split off nuget packages `Microsoft.VisualBasic` and `Microsoft.VisualBasic.My`
- Include this nuget package(s) in all VB templates by default for backwards compatibility

## Why should we do this?
- Other people might want to use this as well, see https://stackoverflow.com/questions/58744/copy-the-entire-contents-of-a-directory-in-c-sharp
- Remove dependencies which are not needed for most scenarious (most of the stuff has some equivalent in .NET BCL)