DPKG source name based on source version value

From https://github.com/anchore/syft/pull/375#discussion_r610635143 , the current extraction of the source name and version depends on the existence of the source version only, and blindly takes the name. See https://github.com/anchore/syft/pull/297/files#diff-194b94d49e66888ab307310de3f49d16f1a446d4194e9c39a62c6d47509d9d3fR113

If this is correct behavior no change needed, however we may need to add a check to see if name should be non-empty.

Start with the DPKG source-name extraction shown in the linked pull-request discussion and referenced file diff, then inspect how source version and name are handled when the name is empty. Reproduce that case and compare the current behavior with the expected DPKG metadata; done means the behavior is decided and covered by an appropriate test.

Written by the indexing model from the issue text.