Privacy: Use relative paths for exported personal data.

Ensures back-compat while moving to paths off of the /exports directory.

Fixes: #44038.

Props: allendav, mrTall, desrosj, garrett-eclipse, cameronamcintyre, nmenescardi, xkon, whyisjake, davidbaumwald.

Built from https://develop.svn.wordpress.org/trunk@48127


git-svn-id: https://core.svn.wordpress.org/trunk@47896 1a063a9b-81f0-0310-95a4-ce76da25c4cd