VSCode default API is Theia API

[tsmaeder]

Bug Description:

When a module require()s vscode, we try to find the plugin the request is made from. If we can't determine the plugin, we return a "default" API. In eclipse-che/che#18219, we're hitting this case. The problem is that we mangle the Theia API to look like the vscode API in the normal case, but not for the default API. This means that in particular vscode.extensions is not defined.

Steps to Reproduce:

t.b.d.

Additional Information

• Operating System:
• Theia Version: