Fix circular import problem w.r.t. detail_levels

Created by: das-g

0181b48f triggered a circular import problem: osmaxx.conversion.converters.detail_levels → osmaxx.conversion.converters.converter_gis (via from osmaxx.conversion.converters.converter_gis.layers import ...) → osmaxx.conversion.converters.converter_gis.gis → osmaxx.conversion.converters.converter_gis.bootstrap → osmaxx.conversion.converters.converter_gis.bootstrap.bootstrap → osmaxx.conversion.converters.detail_levels

f4860e1b solves it by placing module detail_levels in package osmaxx.conversion.converters.converter_gis where it makes more sense than in package osmaxx.conversion.converters, anyway.