Prompt plugin authors to add Swift Package Manager compatibility to their plugin

[vashworth]

When detected that a flutter project is a plugin example app that has a podspec but doesn't have a Package.swift, prompt the user to add Swift Package Manager compatibility and point to documentation with instructions on how to do that.

flutter/packages/flutter_tools/lib/src/plugins.dart

Lines 392 to 418 in 708fe97

	/// Expected path to the plugin's Package.swift. Returns null if the plugin
	/// does not support the [platform] or the [platform] is not iOS or macOS.
	String? pluginSwiftPackageManifestPath(
	FileSystem fileSystem,
	String platform,
	) {
	final String? platformDirectoryName = _darwinPluginDirectoryName(platform);
	if (platformDirectoryName == null) {
	return null;
	}
	return fileSystem.path.join(
	path,
	platformDirectoryName,
	name,
	'Package.swift',
	);
	}
	/// Expected path to the plugin's podspec. Returns null if the plugin does
	/// not support the [platform] or the [platform] is not iOS or macOS.
	String? pluginPodspecPath(FileSystem fileSystem, String platform) {
	final String? platformDirectoryName = _darwinPluginDirectoryName(platform);
	if (platformDirectoryName == null) {
	return null;
	}
	return fileSystem.path.join(path, platformDirectoryName, '$name.podspec');
	}

flutter/packages/flutter_tools/lib/src/project.dart

Line 269 in 708fe97

bool get hasExampleApp => _exampleDirectory(directory).existsSync();

flutter/packages/flutter_tools/lib/src/project.dart

Lines 253 to 257 in 708fe97

	FlutterProject get example => FlutterProject(
	_exampleDirectory(directory),
	_exampleManifest,
	FlutterManifest.empty(logger: globals.logger),
	);

flutter/packages/flutter_tools/lib/src/project.dart

Line 263 in 708fe97

bool get isPlugin => manifest.isPlugin;

[stuartmorgan]

I like it!

(Longer term, we'll want to more aggressively reach long-tail plugin developers by warning end users if they are using a plugin without SPM support, so that we can eventually remove Cocoapods, but that would be probably a couple of releases after SPM support is on stable.)