Use singleton package manager for global API

micropip/__init__.py

@@ -1,19 +1,24 @@
-from ._commands.freeze import freeze
-from ._commands.index_urls import set_index_urls
-from ._commands.install import install
-from ._commands.list import _list as list
-from ._commands.mock_package import (
-    add_mock_package,
-    list_mock_packages,
-    remove_mock_package,
-)
 from ._commands.uninstall import uninstall
+from .package_manager import PackageManager
 
 try:
     from ._version import __version__
 except ImportError:
     pass
 
+singleton_package_manager = PackageManager()
+
+install = singleton_package_manager.install
+set_index_urls = singleton_package_manager.set_index_urls
+list = singleton_package_manager.list
+freeze = singleton_package_manager.freeze
+add_mock_package = singleton_package_manager.add_mock_package
+list_mock_packages = singleton_package_manager.list_mock_packages
+remove_mock_package = singleton_package_manager.remove_mock_package
+
+# TODO: port uninstall
+# uninstall = singleton_package_manager.uninstall
+
 __all__ = [
     "install",
     "list",