MDL-47069 admin: Add option to upload required ZIP at the check page

The plugins check page displays other plugins that the current one
depends on.  Previously, we always displayed the other plugin as a link
to the plugins directory. This did not work well in two scenarios:

* The admin wants/needs to upload the other plugin from the ZIP file
  (e.g. it is not available in the plugins directory),

* the other plugin is actually a standard plugin that is not supposed to
  be registered with the plugins directory.

This patch displays the Install or Upload or Check for updates links
next to the required plugin instead, as appropriate.