fix: keep multiple extras distinct in resolved pins - #297

The resolver keys pins by bare package name (parse_package_key(k).name), so
when a package is pinned with multiple extras (e.g. foo[bar] and foo[baz])
they collapse to a single foo entry and all but the last are silently dropped.

The downstream extras machinery in thin_package_repo.bzl already groups pins by
parts.extra and emits [extra] proxy targets — but it never sees any extras,
because the resolver strips them first. This keeps the extra in the pin key so
that path is actually fed, and references the [extra] proxy target when
rendering all_requirements.

Tested via _test_multi_extra_pins in the lock resolver suite.