This change makes document building --- and specially incremental
document building --- more scalable. The global duplicate-definition
check is handled by a database query, for example.
`setup/private/path-utils'.
The API is a little different: instead of getting the alist and the
path, there's a curried function that gets the alist and produces a
function to do the substitutions.
`setup/path-relativize'.
`setup/path-relativize' is freed from a bunch of things that were due to
historical baggage, but some remain. (Also, update its docs.)
