[flutter_plugin_tools] Introduce a class for packages (#4252)

Packages are the primary conceptual object in the tool, but currently they are represented simply as Directory (or occasionally a path string). This introduces an object for packages and:
- moves a number of existing utility methods into it
- sweeps the code for the obvious cases of using `Directory` to represent a package, especially in method signatures and migrates them
- notes a few places where we should migrate later, to avoid ballooning the size of the PR

There are no doubt other cases not caught in the sweep, but this gives us a foundation both for new code, and to migrate incrementally toward as we find existing code that was missed.

script/tool/test/common/repository_package_test.dart

@@ -0,0 +1,123 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:file/file.dart';
+import 'package:file/memory.dart';
+import 'package:flutter_plugin_tools/src/common/repository_package.dart';
+import 'package:test/test.dart';
+
+import '../util.dart';
+
+void main() {
+  late FileSystem fileSystem;
+  late Directory packagesDir;
+
+  setUp(() {
+    fileSystem = MemoryFileSystem();
+    packagesDir = createPackagesDirectory(fileSystem: fileSystem);
+  });
+
+  group('displayName', () {
+    test('prints packageDir-relative paths by default', () async {
+      expect(
+        RepositoryPackage(packagesDir.childDirectory('foo')).displayName,
+        'foo',
+      );
+      expect(
+        RepositoryPackage(packagesDir
+                .childDirectory('foo')
+                .childDirectory('bar')
+                .childDirectory('baz'))
+            .displayName,
+        'foo/bar/baz',
+      );
+    });
+
+    test('handles third_party/packages/', () async {
+      expect(
+        RepositoryPackage(packagesDir.parent
+                .childDirectory('third_party')
+                .childDirectory('packages')
+                .childDirectory('foo')
+                .childDirectory('bar')
+                .childDirectory('baz'))
+            .displayName,
+        'foo/bar/baz',
+      );
+    });
+
+    test('always uses Posix-style paths', () async {
+      final Directory windowsPackagesDir = createPackagesDirectory(
+          fileSystem: MemoryFileSystem(style: FileSystemStyle.windows));
+
+      expect(
+        RepositoryPackage(windowsPackagesDir.childDirectory('foo')).displayName,
+        'foo',
+      );
+      expect(
+        RepositoryPackage(windowsPackagesDir
+                .childDirectory('foo')
+                .childDirectory('bar')
+                .childDirectory('baz'))
+            .displayName,
+        'foo/bar/baz',
+      );
+    });
+
+    test('elides group name in grouped federated plugin structure', () async {
+      expect(
+        RepositoryPackage(packagesDir
+                .childDirectory('a_plugin')
+                .childDirectory('a_plugin_platform_interface'))
+            .displayName,
+        'a_plugin_platform_interface',
+      );
+      expect(
+        RepositoryPackage(packagesDir
+                .childDirectory('a_plugin')
+                .childDirectory('a_plugin_platform_web'))
+            .displayName,
+        'a_plugin_platform_web',
+      );
+    });
+
+    // The app-facing package doesn't get elided to avoid potential confusion
+    // with the group folder itself.
+    test('does not elide group name for app-facing packages', () async {
+      expect(
+        RepositoryPackage(packagesDir
+                .childDirectory('a_plugin')
+                .childDirectory('a_plugin'))
+            .displayName,
+        'a_plugin/a_plugin',
+      );
+    });
+  });
+
+  group('getExamples', () {
+    test('handles a single example', () async {
+      final Directory plugin = createFakePlugin('a_plugin', packagesDir);
+
+      final List<RepositoryPackage> examples =
+          RepositoryPackage(plugin).getExamples().toList();
+
+      expect(examples.length, 1);
+      expect(examples[0].path, plugin.childDirectory('example').path);
+    });
+
+    test('handles multiple examples', () async {
+      final Directory plugin = createFakePlugin('a_plugin', packagesDir,
+          examples: <String>['example1', 'example2']);
+
+      final List<RepositoryPackage> examples =
+          RepositoryPackage(plugin).getExamples().toList();
+
+      expect(examples.length, 2);
+      expect(examples[0].path,
+          plugin.childDirectory('example').childDirectory('example1').path);
+      expect(examples[1].path,
+          plugin.childDirectory('example').childDirectory('example2').path);
+    });
+  });
+}