feat(side panel): use git repositories to look for GitLab projects

This MR changes the extension side panel. Before it tried to find Git repository in the open workspace folder. After this change, it uses the VS Code Git extension to get all repositories that VS Code recognized.

In the scenario on the screenshot, I opened my workspace directory. The directory itself is not a Git repository but VS Code recognized that four subdirectories are Git repositories.

~/workspace
├── code
├── gitlab-development-kit
├── gitlab-vscode-extension
├── go
├── test-projects
├── tmp
├── weekly-report
└── www-gitlab-com

source control tab	extension before	extension after

How to review?

This MR is split into meaningful commits. I recommend first getting a quick overview of the whole diff (where I left some comments) and then reviewing commit-by-commit.

Related to #345 (closed)