Avoid heavy index imports for pip uninstall & list

pip uninstall and list currently depend on req_install.py which always imports
the expensive network and index machinery. However, it's only in rare situations
that these commands actually hit the network:

- `pip list --outdated`
- `pip list --uptodate`
- `pip uninstall --requirement <url>`

This commit builds on the previous two refactoring commits and modifies these
commands to avoid the expensive imports unless truly necessary.

news/24ca5f01-e27e-41b1-9a9c-7e3a828e22d9.trivial.rst

@@ -0,0 +1,10 @@
+pip uninstall and list currently depend on req_install.py which always imports
+the expensive network and index machinery. However, it's only in rare situations
+that these commands actually hit the network:
+
+- ``pip list --outdated``
+- ``pip list --uptodate``
+- ``pip uninstall --requirement <url>``
+
+This patch refactors req_install.py so these commands can avoid the expensive
+imports unless truly necessary.

src/pip/_internal/commands/list.py

@@ -9,16 +9,15 @@
 from pip._internal.cli.index_command import IndexGroupCommand
 from pip._internal.cli.status_codes import SUCCESS
 from pip._internal.exceptions import CommandError
-from pip._internal.index.collector import LinkCollector
-from pip._internal.index.package_finder import PackageFinder
 from pip._internal.metadata import BaseDistribution, get_environment
 from pip._internal.models.selection_prefs import SelectionPreferences
-from pip._internal.network.session import PipSession
 from pip._internal.utils.compat import stdlib_pkgs
 from pip._internal.utils.misc import tabulate, write_output
 
 if TYPE_CHECKING:
+    from pip._internal.index.package_finder import PackageFinder
     from pip._internal.metadata.base import DistributionVersion
+    from pip._internal.network.session import PipSession
 
     class _DistWithLatestInfo(BaseDistribution):
         """Give the distribution object a couple of extra fields.
@@ -136,11 +135,15 @@ def add_options(self) -> None:
         self.parser.insert_option_group(0, self.cmd_opts)
 
     def _build_package_finder(
-        self, options: Values, session: PipSession
-    ) -> PackageFinder:
+        self, options: Values, session: "PipSession"
+    ) -> "PackageFinder":
         """
         Create a package finder appropriate to this list command.
         """
+        # Lazy import the heavy index modules as most list invocations won't need 'em.
+        from pip._internal.index.collector import LinkCollector
+        from pip._internal.index.package_finder import PackageFinder
+
         link_collector = LinkCollector.create(session, options=options)
 
         # Pass allow_yanked=False to ignore yanked versions.

src/pip/_internal/commands/uninstall.py

@@ -6,7 +6,7 @@
 
 from pip._internal.cli import cmdoptions
 from pip._internal.cli.base_command import Command
-from pip._internal.cli.req_command import SessionCommandMixin
+from pip._internal.cli.index_command import SessionCommandMixin
 from pip._internal.cli.status_codes import SUCCESS
 from pip._internal.exceptions import InstallationError
 from pip._internal.req import parse_requirements

tests/functional/test_cli.py

@@ -57,8 +57,7 @@ def test_entrypoints_work(entrypoint: str, script: PipTestEnvironment) -> None:
     sorted(
         set(commands_dict).symmetric_difference(
             # Exclude commands that are expected to use the network.
-            # TODO: uninstall and list should only import network modules as needed
-            {"install", "uninstall", "download", "search", "index", "wheel", "list"}
+            {"install", "download", "search", "index", "wheel"}
         )
     ),
 )