Avoid vscode reference from discovery.ts

[robertbrignull]

This should fix https://github.com/github/vscode-codeql/security/code-scanning/517

Instead of referencing extLogger directly, we can accept a logger in the constructor. It's a little clunky but I think it's the only way unless we want to remove that logging call.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[charisk]

LGTM!