Extract component functions from search/namespaces

Three functions for extracting namespace symbols from classpaths:

    #'namespaces
    #'namespaces-from-files
    #'namespaces-from-jars

Each function has a unary version, primarily for easier testing, as well
a constant version closed over the current classpath.
Prefer candidates from project namespaces

Addresses the greater half of the TODO item:

;; TODO: prefer things in src/classes to jars