Fix R6 links to superclass methods

[gaborcsardi]

If the superclass is in the a different package. E.g. callr::r_session -> processx::process. (And check that they work if the superclass is in the same package.)

This might need some help from roxygen2.

[hadley]

This already looks ok? https://callr.r-lib.org/reference/r_session.html

Oooooh it's specifically the superclass methods

[hadley]

Example html:

<span class="pkg-link" data-pkg="processx" data-topic="process" data-id="as_ps_handle"><a href="../../processx/html/process.html#method-as_ps_handle"><code>processx::process$as_ps_handle()</code></a></span>

Can use downlit::href_topic() to find the url and then add the fragment from data-id