message: `build-external` now calls code from `build` after input processing
diff: #include "vcpkg_System.h"
  #include "vcpkg_Environment.h"
  #include "vcpkg_Input.h"
  -#include "vcpkglib.h"
  namespace vcpkg::Commands::BuildExternal
  {
  @@ -11,32 +10,11 @@ namespace vcpkg::Commands::BuildExternal
  {
  static const std::string example = Commands::Help::create_example_string(R"(build_external zlib2 C:\path\to\dir\with\controlfile\)");
  args.check_exact_arg_count(2, example);
  + const package_spec spec = Input::check_and_get_package_spec(args.command_arguments.at(0), default_target_triplet, example);
  + Input::check_triplet(spec.target_triplet(), paths);
  + const std::unordered_set<std::string> options = args.check_and_get_optional_command_arguments({});
  - StatusParagraphs status_db = database_load_check(paths);
  -
  - expected<package_spec> maybe_current_spec = package_spec::from_string(args.command_arguments[0], default_target_triplet);
  - if (auto spec = maybe_current_spec.get())
  - {
  - Input::check_triplet(spec->target_triplet(), paths);
  - Environment::ensure_utilities_on_path(paths);
  const fs::path port_dir = args.command_arguments.at(1);
  - const expected<SourceParagraph> maybe_spgh = try_load_port(port_dir);
  - if (auto spgh = maybe_spgh.get())
  - {
  - const Build::BuildResult result = Commands::Build::build_package(*spgh, *spec, paths, port_dir, status_db);
  - if (result != Build::BuildResult::SUCCEEDED)
  - {
  - System::println(System::color::error, Build::create_error_message(result, *spec));
  - System::println(Build::create_user_troubleshooting_message(*spec));
  - exit(EXIT_FAILURE);
  - }
  -
  - exit(EXIT_SUCCESS);
  - }
  - }
  -
  - System::println(System::color::error, "Error: %s: %s", maybe_current_spec.error_code().message(), args.command_arguments[0]);
  - Commands::Help::print_example(Strings::format("%s zlib:x64-windows", args.command));
  - exit(EXIT_FAILURE);
  + Build::perform_and_exit(spec, port_dir, options, paths);
  }
  }
  