Enable some static checking with golangci-lint and fixes a few problems found by the tool

[dlespiau]

Nothing too major found:

• Errors weren't propagated in rpm.ToWFN
• Some misspelled json struct tags
• Some unused fields

I've disabled a few default linting passes, more notably errcheck: there are many code path where we don't check and propagate errors. This will have to be a task for another time.

[dlespiau]

Huum, we seem to be stumbling on golangci/golangci-lint#944

[dlespiau]

A bit more information now:

could not load export data: cannot import "unicode/utf8" (unknown iexport format version 1), export data is newer version - update tool))

The go tools in master must have changed something in how they encode package export data in a way that the latest release of golangci-lint doesn't support yet.

I suggest to mark the master part of the testing matrix, the one testing go master, as not blocking.