Path.BuildSourcea directory is smaller than its descendants
include Comparator.S with type t := tset_extension path ~ext replaces extension of path by ext
Source.t here is a lie in some cases: consider when the context name happens to be "install" or ".alias".
This function does the same as extract_build_context, but has a "righter" type.
set the build directory. Can only be called once and must be done before paths are converted to strings elsewhere.