fix(pypi): normalize extras in requirement strings per PEP 685 (#3588)

## Summary

Extras parsed from requirement strings (e.g., from `requirements.txt`)
were not being normalized, causing mismatches when evaluating PEP 508
marker expressions. For example, `sqlalchemy[postgresql-psycopg2binary]`
would fail to resolve `psycopg2-binary` as a transitive dependency
because the wheel METADATA marker expression `extra ==
"postgresql_psycopg2binary"` uses the underscore-normalized form (per
PEP 685), while the extras set retained the original hyphenated form
from the requirement string.

## Before

```
# requirements.txt
sqlalchemy[postgresql-psycopg2binary]==2.0.36

# Parsed extras: ["postgresql-psycopg2binary"]
# Marker evaluation: "postgresql-psycopg2binary" != "postgresql_psycopg2binary" -> MISS
# Result: psycopg2-binary NOT included as a dependency
```

## After

```
# requirements.txt
sqlalchemy[postgresql-psycopg2binary]==2.0.36

# Parsed extras: ["postgresql_psycopg2binary"]  (normalized)
# Marker evaluation: "postgresql_psycopg2binary" == "postgresql_psycopg2binary" -> MATCH
# Result: psycopg2-binary correctly included as a dependency
```

## Changes

- **`python/private/pypi/pep508_requirement.bzl`**: Apply
`normalize_name()` to each extra during requirement parsing, consistent
with how the package name is already normalized.
- **`tests/pypi/pep508/requirement_tests.bzl`**: Updated existing test
expectation for case normalization and added test case for hyphenated
extras
(`sqlalchemy[asyncio,postgresql-psycopg2binary,postgresql-asyncpg]`).
- **`tests/pypi/pep508/deps_tests.bzl`**: Added
`test_extras_with_hyphens_are_normalized` integration test confirming
that dependencies gated behind hyphenated extras are correctly resolved.
- **`CHANGELOG.md`**: Added entry under Unreleased > Fixed.

Fixes #3587

---------

Co-authored-by: Ignas Anikevicius <240938+aignas@users.noreply.github.com>
(cherry picked from commit 9fe42b1f0badfc258b159dbc0a05a8392c0234e5)