pkg/debuginfo: Mark already existing metadata as uploaded

[brancz]

There can be confusing circumstances, when for example debuginfos have already been uploaded in the past, but there was no metadata. Then the metadata will continue to not be created, while the debuginfo server acknowledges that it already knows about the debuginfos.

This ensures that if the debuginfo server realizes that it already has debuginfos, that it marks them as uploaded in the metadata.

[v-thakkar]

Nice, catch. LGTM!

[brancz]

Long superseeded by lots of other work.