[Export Command] Enabling Manifest Mode

[JavierMatosD]

This PR enables manifest mode for the vcpkg export command.

Summary of changes:

• vcpkg export in manifest mode exports everything in the "vcpkg_installed" directory.
• In manifest mode, the export command emmits an error and fails when port:triplet arguments are provided, as they are not allowed in manifest mode.
• Added a guard to exit with an error message when the installed directory is empty. Previously, it just failed silently.
• Made --output-dir mandatory in manifest mode.

Docs PR: MicrosoftDocs/vcpkg-docs#116

stale

[vicroms]

Suggested change

	if (paths.manifest_mode_enabled())
	{
	auto output_dir_opt = Util::lookup_value(options.settings, OPTION_OUTPUT_DIR);
	// --output-dir is required in manifest mode
	if (auto d = output_dir_opt.get())
	{
	ret.output_dir = paths.original_cwd / *d;
	}
	else
	{
	msg::println_error(msgMissingOption, msg::option = "output-dir");
	Checks::exit_fail(VCPKG_LINE_INFO);
	}
	// Force enable --all-installed in manifest mode
	ret.all_installed = true;
	// In manifest mode the entire installed directory is exported
	if (!options.command_arguments.empty())
	{
	msg::println_error(msgUnexpectedArgument, msg::option = options.command_arguments[0]);
	Checks::exit_fail(VCPKG_LINE_INFO);
	}
	}
	ret.output_dir = ret.output_dir.empty() ? Util::lookup_value(options.settings, OPTION_OUTPUT_DIR)
	.map([&](const Path& p) { return paths.original_cwd / p; })
	auto output_dir_arg = Util::lookup_value(options.settings, OPTION_OUTPUT_DIR);
	if (paths.manifest_mode_enabled())
	{
	if (!output_dir_arg)
	{
	msg::println_error(msgMissingOption, msg::option = "output-dir");
	Checks::exit_fail(VCPKG_LINE_INFO);
	}
	// Force enable --all-installed in manifest mode
	ret.all_installed = true;
	// In manifest mode the entire installed directory is exported
	if (!options.command_arguments.empty())
	{
	msg::println_error(msgUnexpectedArgument, msg::option = options.command_arguments[0]);
	Checks::exit_fail(VCPKG_LINE_INFO);
	}
	}
	ret.output_dir = output_dir_arg.map([&](const Path& p) { return paths.original_cwd / p; }).value_or(paths.root);

[vicroms]

Looks good to me.