Support dependencies between the plugins

[azatsarynnyy]

The VSCode plugins may functionally depend on each other by specifying the extensionDependencies attribute in an plugin's manifest file.
It works as the extensionPack attribute - installs the plugin's dependecnies and additionally it guarantees that the plugins are activated in the right order.

As far as I see such dependency mechanism currently isn't supported by Theia. For now there's no other way to ensure the correct work of the VSCode plugins with the functional dependencies on another plugins.

It's needed for eclipse-che/che#11488. I need to load the OpenShift Connector plugin which depends on the K8s plugin.

Dependency between the plugins is declared here.
OpenShift plugin reuses the TreeDataProvider from K8s plugin here.

Additional info will be attached a bit later.

[azatsarynnyy]

The K8s plugin contributes two views with the corresponding TreeDataProviders

The OpenShift plugin contriutes two views where the Clusters view reuses the TreeDataProvider from the K8s plugin.

[azatsarynnyy]

If I manually add both plugin then all views are empty.

[azatsarynnyy]

I'm closing it since the OpenShift Connector extension no longer depends on the k8s extension.
Feel free to reopen it if there's a need.