Hopefully put this to bed for once and for all. Add a new method
`runtime_formulae_dependencies` which returns the `runtime_dependencies`
which have only the `Formula` objects that exist. I've checked all the
`runtime_dependencies` callers and updated them accordingly.
Also, fix a few cases where runtime dependencies were being read from
the tab when this wasn't desirable.
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.
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