The importlib.metadata and pkg_resources backends unfortunately
normalize extras differently, and we don't really want to continue using
the latter's logic (being partially lossy while still not compliant to
standards), so we add a new abstraction for the purpose.
This removes extra normalization when metadata is loaded into the data
structures, so we can obtain the raw values later in the process during
resolution.
The change in match_markers is needed because this is relied on by the
legacy resolver. Since we removed eager normalization, we need to do
that when the extras are used instead to maintain compatibility.
egg-info distributions may not have the Requires-Dist and Provides-Extra
fields in their metadata. For consistency and to provide an
unsurprising metadata property, we emulate it by reading requires.txt.
A distutils installation is always "flat" (not in e.g. egg form), so
if this distribution's info location is NOT a pathlib.Path (but e.g.
zipfile.Path), it can never contain any distutils scripts.
Tzu-ping Chung
Danny McClanahan
Stéphane Bidoul