Keyword arguments presented by the caller to
``distributions()`` or ``Distribution.discover()``
to narrow the scope of a search for distributions
in all DistributionFinders.
Each DistributionFinder may expect any parameters
and should attempt to honor the canonical
parameters defined below when appropriate.
This mechanism gives a custom provider a means to
solicit additional details from the caller beyond
"name" and "path" when searching distributions.
For example, imagine a provider that exposes suites
of packages in either a "public" or "private" ``realm``.
A caller may wish to query only for distributions in
a particular realm and could call
``distributions(realm="private")`` to signal to the
custom provider to only include distributions from that
realm.