[flutter_plugin_tools] Reimplements the excerpt system inline in the tool, rather than relying on a separate package. (#4417)

* Allows excerpts to come from any package, not just examples.
* Fixes a bug in the excerpting logic that was causing a stray `}` to appear in one example.
* Removes the need for `build.excerpt.yaml` files.
* Remove the dependency on build_runner for excerpts.
* Reduces the time to generate the excerpts from about 10 minutes to about 5 seconds.
* Almost certainly fixes https://github.com/flutter/flutter/issues/107180 (untested).

The new logic is not quite backwards compatible; the `path-base` feature now specifies a real path to the actual source directories, rather than a path into the invisible generated `excerpts/` directory with its special structure. Also, a number of features from the previous package that were not actually used in this repository are no longer supported (such as having multiple section names per `#docregion` pragma).

script/tool/test/readme_check_command_test.dart

@@ -648,7 +648,6 @@ A B C
       final RepositoryPackage package = createFakePackage(
         'a_package',
         packagesDir,
-        extraFiles: <String>[kReadmeExcerptConfigPath],
       );
 
       package.readmeFile.writeAsStringSync('''
@@ -672,40 +671,6 @@ A B C
       );
     });
 
-    test('fails when excerpts are used but the package is not configured',
-        () async {
-      final RepositoryPackage package =
-          createFakePackage('a_package', packagesDir);
-
-      package.readmeFile.writeAsStringSync('''
-Example:
-
-<?code-excerpt "main.dart (SomeSection)"?>
-```dart
-A B C
-```
-''');
-
-      Error? commandError;
-      final List<String> output = await runCapturingPrint(
-          runner, <String>['readme-check', '--require-excerpts'],
-          errorHandler: (Error e) {
-        commandError = e;
-      });
-
-      expect(commandError, isA<ToolExit>());
-      expect(
-        output,
-        containsAllInOrder(<Matcher>[
-          contains('code-excerpt tag found, but the package is not configured '
-              'for excerpting. Follow the instructions at\n'
-              'https://github.com/flutter/flutter/wiki/Contributing-to-Plugins-and-Packages\n'
-              'for setting up a build.excerpt.yaml file.'),
-          contains('Missing code-excerpt configuration'),
-        ]),
-      );
-    });
-
     test('fails on missing excerpt tag when requested', () async {
       final RepositoryPackage package =
           createFakePackage('a_package', packagesDir);