Support referencing auxiliary files when symlinking is used

[springmeyer]

It is not just shapefiles that are multi-file. Rasters, for example, can use world files, which site beside them on the file system and allow gdal to know their extents. If the world file is not also symlinked then it will not be found by gdal and this functionality breaks.

[springmeyer]

Hit another case of this, where the nodata value was stored in an auxiliary file, but due to the symlinking that auxiliary file was left behind and nodata no longer was detected by gdal. Test files at https://github.com/springmeyer/mapnik-bugs/tree/master/nodata-external

[springmeyer]

best solution here would be to stop symlinking.