Explicitly handle incorrect .data paths during wheel install

Previously our wheel installation process allowed wheels which contained
non-conforming contents in a contained .data directory.

After the refactoring to enable direct-from-wheel installation, pip
throws an exception when encountering these wheels, but does not include
any helpful information to pinpoint the cause.

Now if we encounter such a wheel, we trace an error that includes the
name of the requirement we're trying to install, the path to the wheel
file, the path we didn't understand, and a hint about what we expect.

src/pip/_internal/operations/install/wheel.py

@@ -583,7 +583,15 @@ def _install_wheel(
         def make_data_scheme_file(record_path):
             # type: (RecordPath) -> File
             normed_path = os.path.normpath(record_path)
-            _, scheme_key, dest_subpath = normed_path.split(os.path.sep, 2)
+            try:
+                _, scheme_key, dest_subpath = normed_path.split(os.path.sep, 2)
+            except ValueError:
+                message = (
+                    "Unexpected file in {}: {!r}. .data directory contents"
+                    " should be named like: '<scheme key>/<path>'."
+                ).format(wheel_path, record_path)
+                raise InstallationError(message)
+
             scheme_path = scheme_paths[scheme_key]
             dest_path = os.path.join(scheme_path, dest_subpath)
             assert_no_path_traversal(scheme_path, dest_path)

tests/functional/test_install_wheel.py

@@ -681,3 +681,21 @@ def test_correct_package_name_while_creating_wheel_bug(script, package_name):
     package = create_basic_wheel_for_package(script, package_name, '1.0')
     wheel_name = os.path.basename(package)
     assert wheel_name == 'simple_package-1.0-py2.py3-none-any.whl'
+
+
+@pytest.mark.parametrize("name", ["purelib", "abc"])
+def test_wheel_with_file_in_data_dir_has_reasonable_error(
+    script, tmpdir, name
+):
+    """Normally we expect entities in the .data directory to be in a
+    subdirectory, but if they are not then we should show a reasonable error
+    message that includes the path.
+    """
+    wheel_path = make_wheel(
+        "simple", "0.1.0", extra_data_files={name: "hello world"}
+    ).save_to_dir(tmpdir)
+
+    result = script.pip(
+        "install", "--no-index", str(wheel_path), expect_error=True
+    )
+    assert "simple-0.1.0.data/{}".format(name) in result.stderr