Expose targetPackages for buildDotnetModule compatibility

[adampoit]

Problem

Follow-up to #7. mkDotnet now exposes packages and icu, but nixpkgs buildDotnetModule also reads dotnet-sdk.targetPackages when evaluating fetch-deps / runtime inputs:

error: attribute 'targetPackages' missing
at pkgs/build-support/dotnet/build-dotnet-module/default.nix:231:79:
  ++ lib.concatLists (lib.attrValues (lib.getAttrs dotnetRuntimeIds dotnet-sdk.targetPackages));

This still required downstream consumers to add a local shim like:

dotnet-sdk = dotnetSdk // { inherit (dotnetCorePackages.sdk_10_0) targetPackages; };

Solution

Add targetPackages to the buildDotnetModule passthru returned by mkDotnet.

The implementation maps the SDK major version to the corresponding nixpkgs dotnetCorePackages.sdk_<major>_0 package and reuses its targetPackages. If nixpkgs does not provide a matching SDK package, it falls back to {} so the attribute still exists.

Validation

alejandra .
nix run github:nix-community/nix-unit -- --flake '.#tests'

Result: 49/49 unit tests passed.