The code in `AssemblyLoadContext.dll` doesn't need to be in a separate DLL anymore.
S.M.A.dll depends on `AssemblyLoadContext.dll`, so keeping that code out of S.M.A.dll doesn't help make S.M.A smaller size or less dependent. So the code in `AssemblyLoadContext.dll` is moved to `S.M.A.dll` and then we remove `AssemblyLoadContext.dll`.
The changes are:
- Move `CorePsAssemblyLoadContext.cs` to `src\S.M.A\CoreCLR\`
- Update `CorePsAssemblyLoadContext.cs` to get the test took moved to `Utils.InternalTestHooks` and update tests
- Update `build.psm1` and `.csproj` accrodingly
- Update `pwrshcommon.cpp` to remove `AssemblyLoadContext.dll` from the TPA list.
- `S.M.A.AssemblyExtensions` is removed as `PackageManagement` has finished their move to .NET Core 2.0. (I will work with Bryan to get the latest version uploaded to powershell-core)
