Move InitializeSourceRootMappedPaths targets down into common

[clairernovotny]

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

[clairernovotny]

See also dotnet/fsharp#9379

[rainersigwald]

These concepts don't apply to VC++ builds at all, do they? Of course, we have plenty of .NET stuff in common.targets already, but we do try to separate it out. I'd leave it in .Managed, except that really seems to mean VB/C# today.

[clairernovotny]

Do we need another layer of abstraction? (classic solution in computer science 😉)

[ctaggart]

These concepts don't apply to VC++ builds at all, do they?

@rainersigwald, actually they do. Here is the documentation in the readme on Using Source Link in C++ projects. These targets should be moved to common.

[tmat]

I'm not sure if VC++ compiler already supports path mapping but if not it is likely going to at some point to allow reproducible builds. So this logic would make sense for C++ project as well.

[rainersigwald]

There's an undocumented cl.exe flag to trim file paths that's used internally; I don't know if there's a public face for it yet. @olgaark, do you know if there are plans for path-prefix (repo root) stripping for deterministic builds in C++? Could you use the path map that exists today for .NET, or would it need to be different for some reason?