This new version permits omitting the space between the operator and the
boundary in a ruby-style version constraint, like ">1.0.0" instead of
"> 1.0.0".
missingPlugins was hard-coded to work only with provider plugins, so I
renamed it to clarify the usage.
Also renamed a test provider from greater_than to greater-than as the
underscore is an invalid provider name character and this will become a
hard error in the near future.