net/slackcat: How should I cleanup about go.mod?

Hi, I am a maintainer of net/slackcat.

I am trying to cleanup about go.mod.  But I don't know how to fix go.mod, better.
So please review attached my patch.

I consider that there are some issues:
- When go.mod's directory set and run go tidy, then go.sum is update on same directory.
- go.mod and go.sum are required on fetch phase, so I can't override go.mod and go.sum.
- So above, no effect to patch for go.mod and go.sum.  But on build phase, required these.
- I want to update to golang.org/x/sys v0.6.0, but I still have some old versions of it on go.sum.  Really OK?  No problem?

I hope some features to net/slackcat.
- Add support golang.org/x/sys v0.6.0 and/or above.
- Add support minor updating some go modules.

P.S.
I think that I want ${FILESDIR}/go.{mod,sum} support and copy these to ${WRKDIR}
before fetch on USES=go:modules, so more cleanup this issue.  How about?