uv-resolver: only serialize sdist for registry sourced distributions
This fixes an issue in the lock file where, in cases where we had a
non-registry sdist, the information in the sdist was strictly redundant
with the information in the source. This was born out in the code
already where the `sdist` field was only ever used to build a source
distribution type when the source was a registry. In all other cases,
the source distribution data can be materialized from the `source`
field.
