Revert "[package_config] Remove support for .packages files. (#2034)" (#2064)

Author: elliette

pkgs/package_config/CHANGELOG.md

@@ -1,24 +1,5 @@
 ## 2.3.0-wip
 
-- Removes support for the `.packages` file.
-  The Dart SDK no longer supports that file, and no new `.packages` files
-  will be generated.
-  Since the SDK requirement for this package is above 3.0.0,
-  no supporting SDK can use or generate `.packages`.
-
-- Simplifies API that no longer needs to support two separate files.
-  - Renamed `readAnyConfigFile` to `readConfigFile`, and removed
-    the `preferNewest` parameter.
-  - Same for `readAnyConfigFileUri` which becomes `readConfigFileUri`.
-  - Old functions still exists as deprecated, forwarding to the new
-    functions without the `preferNewest` argument.
-
-  Also makes `PackageConfig`, `Package` and `LanguageVersion` `@sealed` classes,
-  in preparation for making them `final` in a future update.
-
-- Adds `PackageConfig.minVersion` to complement `.maxVersion`.
-  Currently both are `2`.
-
 ## 2.2.0
 
 - Add relational operators to `LanguageVersion` with extension methods

pkgs/package_config/README.md

@@ -21,5 +21,6 @@ The primary libraries of this package are
     Just the `PackageConfig` class and other types needed to use
     package configurations. This library does not depend on `dart:io`.
 
-The package no longer contains backwards compatible functionality to
-work with `.packages` files.
+The package includes deprecated backwards compatible functionality to
+work with the `.packages` file. This functionality will not be maintained,
+and will be removed in a future version of this package.