uv-resolver: support unambiguous omission of 'source' and 'version'
When there is only one distribution for a particular package name, any
dependencies (the edges in the resolution graph) that reference that
package name are completely unambiguous. Therefore, we can actually omit
their version and source information and instead derive it from the
distribution entry.
We add some tests to check the success and error cases. That is, when
`source` or `version` are omitted and there are more than one
corresponding distribution for the package name (i.e., it's ambiguous),
then lock deserialization should fail.
