Use pkg_resources.Distribution derived from wheel directly

We now extract all metadata files from the wheel directly into memory
and make them available to the wrapping pkg_resources.Distribution via
the DictMetadata introduced earlier.

Author: chrahunt

src/pip/_internal/distributions/wheel.py

@@ -1,7 +1,8 @@
-from pip._vendor import pkg_resources
+from zipfile import ZipFile
 
 from pip._internal.distributions.base import AbstractDistribution
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+from pip._internal.utils.wheel import pkg_resources_distribution_for_wheel
 
 if MYPY_CHECK_RUNNING:
     from pip._vendor.pkg_resources import Distribution
@@ -16,8 +17,15 @@ class WheelDistribution(AbstractDistribution):
 
     def get_pkg_resources_distribution(self):
         # type: () -> Distribution
-        return list(pkg_resources.find_distributions(
-                    self.req.source_dir))[0]
+        # Set as part of preparation during download.
+        assert self.req.local_file_path
+        # Wheels are never unnamed.
+        assert self.req.name
+
+        with ZipFile(self.req.local_file_path, allowZip64=True) as z:
+            return pkg_resources_distribution_for_wheel(
+                z, self.req.name, self.req.local_file_path
+            )
 
     def prepare_distribution_metadata(self, finder, build_isolation):
         # type: (PackageFinder, bool) -> None

src/pip/_internal/utils/wheel.py

@@ -8,14 +8,18 @@
 from zipfile import ZipFile
 
 from pip._vendor.packaging.utils import canonicalize_name
+from pip._vendor.pkg_resources import DistInfoDistribution
 from pip._vendor.six import PY2, ensure_str
 
 from pip._internal.exceptions import UnsupportedWheel
+from pip._internal.utils.pkg_resources import DictMetadata
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
 if MYPY_CHECK_RUNNING:
     from email.message import Message
-    from typing import Tuple
+    from typing import Dict, Tuple
+
+    from pip._vendor.pkg_resources import Distribution
 
 if PY2:
     from zipfile import BadZipfile as BadZipFile
@@ -29,6 +33,43 @@
 logger = logging.getLogger(__name__)
 
 
+def pkg_resources_distribution_for_wheel(wheel_zip, name, location):
+    # type: (ZipFile, str, str) -> Distribution
+    """Get a pkg_resources distribution given a wheel.
+
+    :raises UnsupportedWheel on any errors
+    """
+    info_dir, _ = parse_wheel(wheel_zip, name)
+
+    metadata_files = [
+        p for p in wheel_zip.namelist() if p.startswith("{}/".format(info_dir))
+    ]
+
+    metadata_text = {}  # type: Dict[str, str]
+    for path in metadata_files:
+        # If a flag is set, namelist entries may be unicode in Python 2.
+        # We coerce them to native str type to match the types used in the rest
+        # of the code. This cannot fail because unicode can always be encoded
+        # with UTF-8.
+        full_path = ensure_str(path)
+        _, metadata_name = full_path.split("/", 1)
+
+        try:
+            metadata_text[metadata_name] = read_wheel_metadata_file(
+                wheel_zip, full_path
+            )
+        except UnsupportedWheel as e:
+            raise UnsupportedWheel(
+                "{} has an invalid wheel, {}".format(name, str(e))
+            )
+
+    metadata = DictMetadata(metadata_text)
+
+    return DistInfoDistribution(
+        location=location, metadata=metadata, project_name=name
+    )
+
+
 def parse_wheel(wheel_zip, name):
     # type: (ZipFile, str) -> Tuple[str, Message]
     """Extract information from the provided wheel, ensuring it meets basic