Avoid using powershell assembly load context with netcoreapp2.0 #3649

Issue Summary

The API AppDomain.GetAssemblies is brought back in .NET Core 2.0 which returns the loaded assemblies from the default loader. Therefore it's possible now for powershell to just depend on the default CoreCLR loader without having our own assembly load context getting in the picture. This would greatly simplify the scenario of hosting powershell in applications.

CoreFX Fixes needed

• https://github.com/dotnet/corefx/issues/18989 - AssemblyLoadContext.GetLoadContext crashes when pass in a dynamic assembly. [Verified Fixed]
• https://github.com/dotnet/corefx/issues/18877 - AppDomain.GetAssemblies doesn't return dynamic assemblies that are emitted on the fly. [Verified Fixed]
• https://github.com/dotnet/corefx/issues/18791 - For an assembly loaded via 'Assembly.LoadFrom', the assembly it referenced cannot be loaded even if they are in the same folder. [Verified Fixed]
• Failure with .NET Core 2.0.0-preview2 SDK dotnet/vscode-csharp#1495 - Preview 2 builds are not yet supported by VS Code C#. [Verified Fixed] -- v1.11.0 VSCode C# extension has been publicly released
• https://github.com/dotnet/corefx/issues/21095 - [Regression] ResourceManager.GetString(string) throw System.IO.FileNotFoundException. [Verified Fixed]

Tasks

• Refactor the code to make PowerShell Core depend on the default CoreCLR loader. Fixed via Refactor PowerShell Core to use the default CoreCLR loader instead #3903
• Update the hosting code in pwrshclrhost.cpp
• Update the host code in Shell.cs in psl-omi-provider (submitted PR Update the entry point method used to call into PowerShell psl-omi-provider#99. PR has been merged and it's verified that PSRP with this fix works with beta.3 powershell)
• Remove Microsoft.PowerShell.CoreCLR.AssemblyExtensions.LoadFrom(string assemblyPath) once the PackageManagement module is migrated to .NET Core 2.0. Tracked by Remove Microsoft.PowerShell.CoreCLR.AssemblyLoadContext.dll from PowerShell core #4149
• Move PowerShell Core to the latest .NET Core to get all fixes we need. (via Move to latest .NET Core and enable -SkipCertificateCheck on OSX #3887 Ran into a regression in the latest .NET Core, so moved back to 2.0.0-preview1 via Move PowerShell back to .NET Core 2.0.0-preview1-002106-00 #4026)
• Think about removing Microsoft.PowerShell.CoreCLR.AssemblyLoadContext.dll completely. Tracked by Remove Microsoft.PowerShell.CoreCLR.AssemblyLoadContext.dll from PowerShell core #4149