Class FileFinder

A helper method for find_file() that looks up a directory for a file we're trying to find. This only creates the Dir Node if it exists on-disk, since if the directory doesn't exist we know we won't find any files in it... :-)

It would be more compact to just use this as a nested function with a default keyword argument (see the commented-out version below), but that doesn't work unless you have nested scopes, so we define it here just so this work under Python 1.5.2.

find_file(str, [Dir()]) -> [nodes]

filename - a filename to find
paths - a list of directory path *nodes* to search in.  Can be
        represented as a list, a tuple, or a callable that is
        called with no arguments and returns the list or tuple.

returns - the node created from the found file.

Find a node corresponding to either a derived file or a file
that exists already.

Only the first file found is returned, and none is returned
if no file is found.