fix: Backport, fixed `found-at` with absolute path

- Refs SassDoc/gulp-sassdoc#2

See
abe4fd3ef8d7a3540446b29e041c6214d4618ff0
74d34316ff22f968cc0b4ea89f253486b9a96891
2c970528c71d94c18d9d1e758e19fda2a0dcbace