checklist for v3.0

[majewsky]

The last release was over three years ago, so we have a bit of ecosystem evolution to catch up to:

• switch from golangvend to go.mod
• switch from Travis CI to GitHub Actions
• check if we can get rid of gocovcat (Go 1.10 can generate coverprofiles for multiple packages at once) We have at least one testcase (the one with the ALPM hook where we absolutely have to run the test binary manually.

Followup issues from adding generators:

• holo scan as non-privileged fails because regular files copied from /usr/share/holo cannot have all their original permissions applied by a non-privileged user

Also, some more things I want to do for v3.0 that I don't feel like opening separate issues for:

• add CLI selectors for generators (e.g. holo apply /usr/share/holo/generators/foo.sh applies all entities with source files produced by that generator)
• double-check shell completions (zsh completion looks broken for me right now)
• add holo selectors to simplify shell completion implementations
• do not litter /tmp/holo-ssh-keyNNN files
• merge Run tests in parallel, automatically detect which packages need go test #46 (or at least the parts that still apply)

After the release:

• refresh the website: proofread and update for new features where necessary
• include the manpages in the website
• render manpages from Markdown instead of POD (I wanted to make my life easier by going with a more common format, but Markdown is not expressive enough to make nice manpages out of it. I'd rather write a POD parser for the website than try to shoehorn custom syntax into Markdown.)
• update the Pacman repo
• bring the Debian repo back in shape
• do a "Show HN"