Dart package:name/path.dart imports have no resolution rule (import recall 81% on dart-lang/http) #3597

Problem

A Dart import 'package:http_profile/http_profile.dart' names <the http_profile package>/lib/http_profile.dart. The engine has no rule for the package: scheme. The token goes to the name search, and in a monorepo where the same file name exists in several packages, that search draws nothing.

Measured

tests/tools/import_graph_accuracy.py dart (dart-lang/http, added in #3594): precision 99.1%, recall 80.8%, 143 missed imports. For example, cupertino_http/lib/src/cupertino_client.dart → pkgs/http_profile/lib/http_profile.dart.

Fix direction

Treat package:name/rest as a path mirror: the target is a file whose path ends in name/lib/rest. Dart packages sit in a directory named after the package by convention (pkgs/http_profile/), so no pubspec parsing is needed, the same spirit as the JS/TS alias mirror in #3589. dart: stays external.

The 5 false positives here are a separate, cross-language issue: an import written inside a /// doc comment is captured.