Windows: Fails for files in root folder

Surfaced in:
https://github.com/mapbox/mapbox-studio/issues/1344

mapnik-omnivore cannot detect the file type, if the file is located in the root of the drive, e.g. directly in `C:\`.
I've tested and this is **not** an issue on Linux with files in `/`.

For csv it fails with [`Incompatible filetype`](https://github.com/mapbox/mapnik-omnivore/blob/v4.1.0/index.js#L79).

For geojson it fails with [`Error obtaining layer names`](https://github.com/mapbox/mapnik-omnivore/blob/v4.1.0/lib/datasource-processor.js#L648)

This does not happen if the file is moved into a sub directory.

Start with index.js around line 79 and lib/datasource-processor.js around line 648, then reproduce the CSV and GeoJSON failures using files directly under a Windows drive root. Done means mapnik-omnivore detects both file types there while continuing to work for files in subdirectories and on Linux.

Written by the indexing model from the issue text.