Incorrect handling of backslashes in RECORD file by importlib.metadata

Bug report

Bug description:

The class importlib.metadata.PackagePath does not correctly handle backslashes in the RECORD dist-info file.
According to the specs, backslashes are allowed:

The path may be either absolute, or relative to the directory containing the .dist-info directory (commonly, the site-packages directory). On Windows, directories may be separated either by forward- or backslashes (/ or \).

This leads to PackagePath.name wrongfully returning the entire path instead of just the filename.

Steps for Reprocution

As an example, take ruff 0.14.14 on Windows

..\..\Scripts\ruff.exe,sha256=HJzLwAeBnSdcuXo602XCKCTuDMMrn5dZPIEzmZ0w6QY,31781376
ruff-0.14.14.dist-info/INSTALLER,sha256=5hhM4Q4mYTT9z6QB6PGpUAW81PGNFrYrdXMj4oM_6ak,2
ruff-0.14.14.dist-info/REQUESTED,sha256=47DEQpj8HBSa-_TImW-5JCeuQeRkm5NMpJWZG3hSuFU,0
ruff-0.14.14.dist-info\METADATA,sha256=QD17c7tQSny5DBVcDewbxp32FVd0m05EfaH_AQvKuhQ,26651
ruff-0.14.14.dist-info\RECORD,,
ruff-0.14.14.dist-info\WHEEL,sha256=jsSEiVNsW1dJj5gDaReR40i7mhgBjWtms6nAD6EViXU,94
ruff-0.14.14.dist-info\licenses\LICENSE,sha256=JZfYVBIrd93HGXFWTKI1CjdghXXOMkrcVlCisgUcjxg,20731
ruff\__init__.py,sha256=47DEQpj8HBSa-_TImW-5JCeuQeRkm5NMpJWZG3hSuFU,0
ruff\__main__.py,sha256=w1Ehfzzy8tMM7r7dFrKmi3DkmTZyV_xJ40K_sxoO6X0,2908

pip install "ruff==0.14.14"

import importlib.metadata
from pathlib import Path

for pkg in importlib.metadata.distributions():
    if pkg.name == "ruff":
        for file in pkg.files:
            print(file.name, Path(file).name)

Expected Output

ruff.exe ruff.exe
INSTALLER INSTALLER
REQUESTED REQUESTED
METADATA METADATA
RECORD RECORD
WHEEL WHEEL
LICENSE LICENSE
__init__.py __init__.py
__main__.py __main__.py

Actual Output

..\..\Scripts\ruff.exe ruff.exe
INSTALLER INSTALLER
REQUESTED REQUESTED
ruff-0.14.14.dist-info\METADATA METADATA
ruff-0.14.14.dist-info\RECORD RECORD
ruff-0.14.14.dist-info\WHEEL WHEEL
ruff-0.14.14.dist-info\licenses\LICENSE LICENSE
ruff\__init__.py __init__.py
ruff\__main__.py __main__.py

CPython versions tested on:

3.14

Operating systems tested on:

Windows

Linked PRs

• gh-149540

Comienza con importlib.metadata.PackagePath y reproduce el caso de Windows utilizando el contenido de RECORD y el ejemplo del issue. Se considera terminado cuando PackagePath.name devuelve únicamente el nombre del archivo para las rutas separadas por barras invertidas, mientras que los casos mostrados con barras diagonales siguen produciendo los nombres esperados.