Search sample/prelude-modules.el in prelude-dir during init

[fapdash]

If the user installed prelude to different directory via the PRELUDE_INSTALL_DIR variable then the sample file won't be found in the user-emacs-directory.

I'm currently in the process of updating the docs for the Exercism Emacs Lisp track and installed prelude as it is recommended on there and ran into this issue.

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• [x] The commits are consistent with our contribution guidelines
• [x] You've updated the changelog (if adding/changing user-visible functionality)
• [x] You've updated the user manual (if adding/changing user-visible functionality like modules, commands, configuration options, etc)

Thanks!

[bbatsov]

Thanks!