Some features do not exist in set locations, but can be anywhere in the layer.
This allows those featurefmt to specify the filenames to be scanned by
regexp, as opposed to purely by path prefix.
If any current users of this express paths which use regexp special characters
this could be a breaking change for them (with the exception of . which will
continue to work as it matches against the literal character .), however
none of the code in this repo does that.
fixes#456
