MDL-47069 admin: Add option to upload required ZIP at the plugins 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.