Use singleton package manager for global API (#141)

* Use singleton package manager for global apis

* import name

* Fix test

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Fix type

* Fix another test

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Fix parameter order

* Rename

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

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
 
+_package_manager_singleton = PackageManager()
+
+install = _package_manager_singleton.install
+set_index_urls = _package_manager_singleton.set_index_urls
+list = _package_manager_singleton.list
+freeze = _package_manager_singleton.freeze
+add_mock_package = _package_manager_singleton.add_mock_package
+list_mock_packages = _package_manager_singleton.list_mock_packages
+remove_mock_package = _package_manager_singleton.remove_mock_package
+
+# TODO: port uninstall
+# uninstall = _package_manager_singleton.uninstall
+
 __all__ = [
     "install",
     "list",