Design Package Source mapping experiences for dotnet package

[nkolev92]

Child of #10730

This covers the package source mapping commands for the dotnet add package command (or maybe even, dotnet update package).

[JonDouglas]

This should be done

[nkolev92]

@JonDouglas do you have a pointer to the specs for this?

I don't think we have specs for things like dotnet add package etc.
If source mapping is enabled, for example we may need a --source parameter to know wh at to map to.

[nkolev92]

Reopening this as dotnet add package doesn't respect it yet.