Don't drop .rb suffix for formula names

[vladshablinsky]

Fixes #1718

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[MikeMcQuaid]

Thanks @vladshablinsky for the fix and @jasonkarns for the report (and apologies for overestimating the complexity and edge-cases here).

[jasonkarns]

Thanks @vladshablinsky!! And no worries @MikeMcQuaid. I don't begrudge your maintainership (and thank you!)