Move InitializeSourceRootMappedPaths targets down into common

The targets for initializing `SourceRoot` items for deterministic builds lives in the Managed.core targets today. This means that other languages, like F#, need to copy the targets, which will lead to divergence and issues.

https://github.com/dotnet/roslyn/blob/master/src/Compilers/Core/MSBuildTask/Microsoft.Managed.Core.targets#L158

We should move these targets down into the common targets so they're available to all languages as deterministic builds is a desirable, and encouraged, for published libraries and applications.

@tmat @cartermp @ctaggart