Let geojson files in mapping be specified relative to mapping files

[pnorman]

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Context

Right now when specifying a geojson file for geojson_intersects_feature it needs to be specified as an absolute path or relative to the imposm binary. This makes it difficult to distribute a mapping.yaml file with associated geojson in a way that doesn't depend on deployment-specific paths.

Expected Behavior

A relative path in a mapping file is relative to the mapping file.

Possible Fix

Change the current working directory before https://github.com/omniscale/imposm3/blob/3b6e6b38f4529ac468580040dff25a0b47a037c9/mapping/columns_intersection.go#L41