Create plugin symlinks for Windows and Linux (#50599)

This makes ephemeral symlinks to each plugin, for use by build systems.
This is similar to the logic implemented in the Podfile on iOS and
macOS, but managed internally to the Flutter tool.

Exploration for addressing #32719 and #32720
Related to #41146

packages/flutter_tools/lib/src/project.dart

diff --git a/packages/flutter_tools/lib/src/project.dart b/packages/flutter_tools/lib/src/project.dart
index d227d8e..6570e91 100644
--- a/packages/flutter_tools/lib/src/project.dart
+++ b/packages/flutter_tools/lib/src/project.dart
@@ -963,6 +963,9 @@
   /// Ideally this will be replaced in the future with inspection of the project.
   File get nameFile => ephemeralDirectory.childFile('exe_filename');
 
+  /// The directory to write plugin symlinks.
+  Directory get pluginSymlinkDirectory => ephemeralDirectory.childDirectory('.plugin_symlinks');
+
   Future<void> ensureReadyForPlatformSpecificTooling() async {}
 }
 
@@ -997,6 +1000,9 @@
   /// the build.
   File get generatedMakeConfigFile => ephemeralDirectory.childFile('generated_config.mk');
 
+  /// The directory to write plugin symlinks.
+  Directory get pluginSymlinkDirectory => ephemeralDirectory.childDirectory('.plugin_symlinks');
+
   Future<void> ensureReadyForPlatformSpecificTooling() async {}
 }