It's not possible to determine the difference between a non-keg-only
keg that failed to link and one that wasn't linked by us intentionally.
To avoid additional complexity of storing this logic in another place
let's back out this relatively new functionality; sticky keg-only links
is a better and more desirable behaviour anyway.
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.