Don't unpack wheel files after building for install

Actual installation has been using the wheel file directly for some
time. The last piece that required an unpacked wheel was metadata. Now
that it uses the wheel file directly, we can remove the unpacking after
build.

Author: chrahunt

src/pip/_internal/wheel_builder.py

@@ -13,13 +13,11 @@
 from pip._internal.operations.build.wheel import build_wheel_pep517
 from pip._internal.operations.build.wheel_legacy import build_wheel_legacy
 from pip._internal.utils.logging import indent_log
-from pip._internal.utils.marker_files import has_delete_marker_file
 from pip._internal.utils.misc import ensure_dir, hash_file
 from pip._internal.utils.setuptools_build import make_setuptools_clean_args
 from pip._internal.utils.subprocess import call_subprocess
 from pip._internal.utils.temp_dir import TempDirectory
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
-from pip._internal.utils.unpacking import unpack_file
 from pip._internal.utils.urls import path_to_url
 from pip._internal.vcs import vcs
 
@@ -313,27 +311,6 @@ def build(
                     req.link = Link(path_to_url(wheel_file))
                     req.local_file_path = req.link.file_path
                     assert req.link.is_wheel
-                    if should_unpack:
-                        # XXX: This is mildly duplicative with prepare_files,
-                        # but not close enough to pull out to a single common
-                        # method.
-                        # The code below assumes temporary source dirs -
-                        # prevent it doing bad things.
-                        if (
-                            req.source_dir and
-                            not has_delete_marker_file(req.source_dir)
-                        ):
-                            raise AssertionError(
-                                "bad source dir - missing marker")
-                        # Delete the source we built the wheel from
-                        req.remove_temporary_source()
-                        # set the build directory again - name is known from
-                        # the work prepare_files did.
-                        req.source_dir = req.ensure_build_location(
-                            self.preparer.build_dir
-                        )
-                        # extract the wheel into the dir
-                        unpack_file(req.link.file_path, req.source_dir)
                     build_successes.append(req)
                 else:
                     build_failures.append(req)