Function: exists_as_package

--Usage--
> exists_as_package("package")

Check if a package exists in the data folder.

Returns the package's version number as major-minor-patch, or nil if the package does not exists.

--Parameters--
! Parameter	Type			Description
| @input@	[[type:string]]		Full name of the package folder, including game and extension.

--Examples--
> exists_as_package("magic-modules.mse-include")  ==  "2026-02-28"
> exists_as_package("magic-quadruple-faced-leveler.mse-style")  ==  nil