Store ELF requirements in METADATA Requires-External fields

[tiran]

Python core metadata has a multi-use field Requires-External to specify external requirements. We can use the field to store external requirements such as ELF requires directly in the metadata file. This eliminates the need to maintain a separate file for Fromager ELF requirements.

As far as I can tell, the value of Requires-External fields are pretty much free-form. The field can contain environment markers after a semicolon, but we don't need it. The platform is implied by the wheel file name. We have to update the Dynamic field if-and-only-if it is present.