Remove support for .pub package-local cache (#3823)

lib/src/entrypoint.dart

diff --git a/lib/src/entrypoint.dart b/lib/src/entrypoint.dart
index d18f6e2..1b98da6 100644
--- a/lib/src/entrypoint.dart
+++ b/lib/src/entrypoint.dart
@@ -206,22 +206,10 @@
 
   /// The path to the entrypoint package's `.dart_tool/pub` cache directory.
   ///
-  /// If the old-style `.pub` directory is being used, this returns that
-  /// instead.
-  ///
   /// For globally activated packages from path, this is not the same as
   /// [configRoot], because the snapshots should be stored in the global cache,
   /// but the configuration is stored at the package itself.
-  String get cachePath {
-    if (isGlobal) {
-      return globalDir!;
-    } else {
-      var newPath = root.path('.dart_tool/pub');
-      var oldPath = root.path('.pub');
-      if (!dirExists(newPath) && dirExists(oldPath)) return oldPath;
-      return newPath;
-    }
-  }
+  String get cachePath => globalDir ?? root.path('.dart_tool/pub');
 
   /// The path to the directory containing dependency executable snapshots.
   String get _snapshotPath => p.join(cachePath, 'bin');
@@ -444,8 +432,6 @@
 
   /// Precompiles all [_builtExecutables].
   Future<void> precompileExecutables() async {
-    migrateCache();
-
     final executables = _builtExecutables;
 
     if (executables.isEmpty) return;
@@ -914,26 +900,6 @@
     }
   }
 
-  /// If the entrypoint uses the old-style `.pub` cache directory, migrates it
-  /// to the new-style `.dart_tool/pub` directory.
-  void migrateCache() {
-    // Cached packages don't have these.
-    if (isCached) return;
-
-    var oldPath = p.join(_configRoot!, '.pub');
-    if (!dirExists(oldPath)) return;
-
-    var newPath = root.path('.dart_tool/pub');
-
-    // If both the old and new directories exist, something weird is going on.
-    // Do nothing to avoid making things worse. Pub will prefer the new
-    // directory anyway.
-    if (dirExists(newPath)) return;
-
-    ensureDir(p.dirname(newPath));
-    renameDir(oldPath, newPath);
-  }
-
   /// We require an SDK constraint lower-bound as of Dart 2.12.0
   ///
   /// We don't allow unknown sdks.

lib/src/executable.dart

diff --git a/lib/src/executable.dart b/lib/src/executable.dart
index a80437a..accde6e 100644
--- a/lib/src/executable.dart
+++ b/lib/src/executable.dart
@@ -67,8 +67,6 @@
     }
   }
 
-  entrypoint.migrateCache();
-
   var snapshotPath = entrypoint.pathOfExecutable(executable);
 
   // Don't compile snapshots for mutable packages, since their code may