Move plugin data path to project root

[jbudz]

From #7457 (comment)

Currently, Kibana installs external plugins to installedPlugins and plugin data to installedPlugins/.data. The data path implies that this is only meant for external plugins. Moving this to the root directory will give all plugins a location to write to.

[jbudz]

If this is moved to the root directory, does it make sense to prefix with a . still?

[w33ble]

I'm in favor of moving this to the root, or at least out of installedPlugins, as making the distinction between internal and external plugins is odd. It also creates more work as we'd require 2 data paths, and if we want to take something external and make it internal, we have to update the plugin to use the "right" path.

As for removing the . prefix, it's not so much about hiding it as it is about not concerning users with it, so I could go either way.