Improve feedback from @inherits and friends

[hadley]

And considerably reduce the scope of link tweaking.

Fixes #1135.

[hadley]

@gaborcsardi do you think it's safe to just remove all of find_topic_filename and friends?

[gaborcsardi]

I will take a better look tomorrow to be sure.

[gaborcsardi]

Looks great!

[gaborcsardi]

@hadley Re find_topic_filename() (and friends), I tried to make it a no-op:

find_topic_filename <- function(pkg, topic, tag = NULL) {
  return(topic)
}

and this doesn't really break anything, except that we need to update/remote 2-3 test cases. I checked on a two days old R-devel as well.

OTOH there are a couple of things to consider.

1. It does change the generated manual of course. E.g. for ggplot2:

   ❯ git diff man/ | wc -l
        437
   

   so this will be a bit disrupting.

2. There is a slight, but non-zero chance that a link breaks because the escaping of special characters in topic names works out differently if we don't rewrite the links. This is just a theoretical possibility to keep in mind if we see broken links, so definitely not a major issue.

3. Currently it is nice to get a warning from roxygen if you try to link to a non-existent topic. If we remove the link lookup, then this goes away, and you only learn about broken links from R CMD check (unless you turn off the rd xrefs check of course, which you might). So even if we don't rewrite the links, it might make sense to still do the link lookup, and warn for broken links.

[hadley]

Yeah, so maybe just easier to leave as is for now.