feat: implement DUB module extension and support for library packages

[dcarp]

This PR adds first-class DUB dependency support to rules_d for both bzlmod and WORKSPACE users.

• Adds DUB module extension support:

  • dub.from_dub_selections(...) reads dub.selections.lock.json
  • Registers DUB packages as an external repo (default @dub)

• Adds WORKSPACE API:

  • d_register_dub_repository(...) in dub/repositories.bzl

• Adds lockfile tooling:

  • dub_lock_dependencies(...) macro
  • dub/selections_lock/generate_selections_lock.d generator

• Adds/adjusts D tool exposure needed by lock generation:

  • @rules_d//d:dub and related wiring

• Refactors DUB internals:

  • Moves package fetch/build-file logic into dub/private
  • Moves curl/integrity helpers under dub/private

• Adds e2e smoke coverage for DUB usage:

  • bzlmod setup in e2e/smoke/MODULE.bazel
  • WORKSPACE setup in e2e/smoke/WORKSPACE.bazel
  • test using @dub//semver

  Implements Add dub_package repository rule #1