`PluginContext.globalStorage" broken in self-hosting context.

[tsmaeder]

Describe the bug

When running a self-hosting workspace according to https://github.com/eclipse/che-theia/blob/master/CONTRIBUTING.md, the value of theia.PluginContext.globalStoragePath is wrong: it ends up being /home/theia/home/theia- which is incorrect.

When a plugin is trying to write a file to that location using the vscode.workspace.fs API, the write will fail. The latest version of the ms python extension shows the problem

Che version

master

Steps to reproduce

Start a self-hosting workspace containing the ms python extension. You'll find the following error in the plugin host log:

 (FileSystemError): Error: EACCES: permission denied, mkdir '/home/theia'
    at new FileSystemError (/projects/theia/packages/plugin-ext/lib/plugin/types-impl.js:1294:28)

[tsmaeder]

There are two problems going on here:

1. We assume that the home directory of the container running theia is /home/theia, while in the theia-dev container it's '/home/theia-dev'
2. When a plugin uses the vscode file system API, file:// uris will end up writing to the theia back-end container, not the plugin container.

Approaches to fix this would be to share the configuration locations across containers or to rewrite plugin-local uri's

[apupier]

I think that sharing the configuration location across containers should allow to support Java Language support for Camel K files #16018
(to put weigh in this option)