Function: exists_in_package

--Usage--
> exists_in_package("/package/filename")

Check if a file exists in a given package.

--Parameters--
! Parameter	Type			Description
| @input@	[[type:string]]		Path of the file, starting from the data directory.

--Examples--
> exists_in_package("/magic-modules.mse-include/watermarks/custom_user_watermark.png")  ==  true
