[Feature Request] Build dotnet-runtime-deps packages

[Dominik Viererbe]

Microsoft builds and recommends [1] building the packages `dotnet-runtime-deps-6.0`, `dotnet-runtime-deps-7.0`, `dotnet-runtime-deps-8.0`. These contain the dependencies needed by the .NET runtime for the specific version.

This is useful if a developer created a self contained application that is bundled with a runtime, because the bundled runtime still has dependencies [2]. Having these packages improves the developer experience, because developers do not need to install (& keep track of changes to) runtime dependencies manually.

[Implementation details]

I already looked into how we would add these packages:

- `dotnet-runtime-deps-X.0` would just be an empty package
- `dotnet-runtime-X.0` would depend on `dotnet-runtime-deps-X.0`
- the `libicuXX` depends would move to `dotnet-runtime-deps-X.0`
- We need to capture the ${shlibs:Depends} of `dotnet-runtime-X.0` and copy it

I looked up that dh_shlibdeps generates `debian/*.substvars` where * is the package name. The .substvars file will contain the substitution values. Then we just need to "smartly" copy the values over from `dotnet-runtime-X.0.substvar` into `dotnet-runtime-deps-X.0.substvar` before calling dh_gencontrol in override_dh_gencontrol.

[1] https://learn.microsoft.com/en-us/dotnet/core/distribution-packaging#recommended-packages
[2] https://learn.microsoft.com/en-us/dotnet/core/install/linux-ubuntu-install?pivots=os-linux-ubuntu-2404&tabs=dotnet8#dependencies