This is the TODO list for the python rewrite

build from file
build from http
dont allow abspaths
check urls
urls we want:
relative - require base url set.
file:// - autodetect ?
urls to non arch paths.
arch:// - use pybaz ?
add ConfigSource type
implement a build
implement a front end
ConfigEntry eq with bad types, or with different values
add svn, cvs, tarball support back in.
map urls to other urls to allow local overrides;
	requires a syntax for mapping files
	and a ui to select one - all the other machinery is done.
map urls and try all variations in some order - giving us search paths.
map a full url to different url
map a prefix url to a result with the tail preserved.
adding a non-'str' mapping -> assertion (urls are all plain strings)
adding an empty length mapping -> assertion.
the longest matching prefix is chosen.
