context:
  * URL is extracted from an "href" attribute
  * URL path either ends with:
    - path separator
    - basename with no file extension
    - basename with file extension that is included in a whitelist

this determination is necessary for:
  * separating page prerequisites from page recursion
    - important when the max recursion depth has been reached
  * mapping the URL to a file path
    - important when applying options:
        --default-page
        --adjust-extension