This is passing against 21.1.3, but not main. The installed-files.txt
test case is substentially rewritten to correctly test against a legacy
setuptools installation.

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.