fix(exthost): Fix error when trying to read package metadata

[bryphe]

Issue: The python fails with an error when trying to read the extension metadata via the vscode.extensions API, here:

    const extensions = (require('vscode') as typeof import('vscode')).extensions;
    // tslint:disable-next-line:no-non-null-assertion
    const extension = extensions.getExtension(extensionId)!;
    // tslint:disable-next-line:no-unsafe-any
    const extensionVersion = extension.packageJSON.version;

Defect: The extension IDs we were sending to the extension host were improperly constructed. To be in sync with the extension host, they should be of the form publisher.name.

Fix: Correctly construct the extension ID.

This gets us to the next round of blockers of the Python extension - it requires the workspace.fs API to implemented (it uses these operations to look for the .env / virtual environment setup).