fix(metadata): reject colliding extra names when writing

[vera-rubin]

With provides_extra=["dev_test", "dev-test"], Metadata.as_rfc822() writes two identical Provides-Extra: dev-test headers. The core metadata specification requires writers to raise an error when extra names clash after normalization.

from packaging.metadata import Metadata

metadata = Metadata.from_raw({
    "metadata_version": "2.1", "name": "example", "version": "1.0",
    "provides_extra": ["dev_test", "dev-test"],
})
metadata.as_rfc822()

We now raise InvalidMetadata for duplicate normalized extra names in the writer. The regression test fails before the fix and passes after it, and the existing test suite passes.

Tested base: ef837aad3127167671eb37b9f6bd8a94ec7ce26e (packaging 26.4.dev0).

— ChipLab