CQ. In summarizePackage() process only lib/ directory.

When I'm doing breaking changes, I put generatons into their own
analysis context, to use a stable version of the analyzer. All these
generators are in tool/, so never API.

Change-Id: Ic0488a7f2c8111955de685dbd2cb170cfbb21274
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/497421
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer_utilities/lib/src/api_summary/summarize_package.dart

@@ -28,9 +28,10 @@ Future<String> summarizePackage(
   ApiSummaryCustomizer Function()? createCustomizer,
 }) async {
   var provider = PhysicalResourceProvider.INSTANCE;
+  var libPath = provider.pathContext.join(packagePath, 'lib');
   var collection = AnalysisContextCollection(
-    includedPaths: [packagePath],
     resourceProvider: provider,
+    includedPaths: [libPath],
   );
   // Use `.single` to make sure that `collection` just contains a single
   // context. This ensures that `publicApi.build` will see all the files in