Allow the option of not overriding package-user-dir

[dmgerman]

Currently prelude overrides user-package-dir to be /elpa

This patch does not change that behaviour, but gives the user to disable it.

Adds a variable prelude-override-package-user-dir that by default is t.

The path is only set when prelude-override-package-user-dir is true

Addresses #1406

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