Correctly normalize paths relative to install path

Since the legacy installed-files.txt writes paths relatively to the
egg-info directory, we need to introduce a new property on
BaseDistribution to return that directory's path (analoguous to
pkg_resources's Distribution.egg_info).

Entries in RECORD are normalized with pathlib.Path so they have the
correct path component separator depending on the platform (e.g. '/' on
Windows), to match the previous behavior.

src/pip/_internal/commands/show.py

@@ -1,6 +1,6 @@
 import csv
 import logging
-import os
+import pathlib
 from optparse import Values
 from typing import Iterator, List, NamedTuple, Optional
 
@@ -102,12 +102,17 @@ def _files_from_record(dist: BaseDistribution) -> Optional[Iterator[str]]:
             return None
         return (row[0] for row in csv.reader(text.splitlines()))
 
-    def _files_from_installed_files(dist: BaseDistribution) -> Optional[Iterator[str]]:
+    def _files_from_legacy(dist: BaseDistribution) -> Optional[Iterator[str]]:
         try:
             text = dist.read_text('installed-files.txt')
         except FileNotFoundError:
             return None
-        return (p for p in text.splitlines(keepends=False) if p)
+        paths = (p for p in text.splitlines(keepends=False) if p)
+        root = dist.location
+        info = dist.metadata_directory
+        if root is None or info is None:
+            return paths
+        return (str(pathlib.Path(info, p).resolve().relative_to(root)) for p in paths)
 
     for query_name in query_names:
         try:
@@ -121,11 +126,11 @@ def _files_from_installed_files(dist: BaseDistribution) -> Optional[Iterator[str
         except FileNotFoundError:
             entry_points = []
 
-        files_iter = _files_from_record(dist) or _files_from_installed_files(dist)
+        files_iter = _files_from_record(dist) or _files_from_legacy(dist)
         if files_iter is None:
             files: Optional[List[str]] = None
         else:
-            files = sorted(os.path.relpath(p, dist.location) for p in files_iter)
+            files = sorted(files_iter)
 
         metadata = dist.metadata
 

src/pip/_internal/metadata/base.py

@@ -60,6 +60,18 @@ def location(self) -> Optional[str]:
         """
         raise NotImplementedError()
 
+    @property
+    def metadata_directory(self) -> Optional[str]:
+        """Location of the metadata directory.
+
+        Similarly to ``location``, a string value is not necessarily a
+        filesystem path. ``None`` means the distribution is created in-memory.
+
+        For a modern .dist-info installation on disk, this should be something
+        like ``{location}/{raw_name}-{version}.dist-info``.
+        """
+        raise NotImplementedError()
+
     @property
     def canonical_name(self) -> "NormalizedName":
         raise NotImplementedError()

src/pip/_internal/metadata/pkg_resources.py

@@ -48,6 +48,10 @@ def from_wheel(cls, path: str, name: str) -> "Distribution":
     def location(self) -> Optional[str]:
         return self._dist.location
 
+    @property
+    def metadata_directory(self) -> Optional[str]:
+        return self._dist.egg_info
+
     @property
     def canonical_name(self) -> "NormalizedName":
         return canonicalize_name(self._dist.project_name)