Issues with not case/fragment-normalizing file paths (macOS, Windows)

Problem

In VS Code we deal with resource `URIs` across all layers. In most cases we use `URI.toString()` to decide if a resource is equal to another resource. This assumption is not very good when we talk about file system paths because on macOS and Windows a file path should be considered equal if the paths are identical, even if the casing differs (`/some/path` === `/SOME/path`). Only Linux has a case-sensitive file system where this rule does not apply. This leads to numerous related issues: - [x] Mac: chokidar file watcher does not work if you run with wrong casing (https://github.com/Microsoft/vscode/issues/1426) - [ ] quick open and editors can show the same file multiple times (https://github.com/Microsoft/vscode/issues/17967, https://github.com/Microsoft/vscode/issues/15120, https://github.com/Microsoft/vscode/issues/18718, https://github.com/Microsoft/vscode/issues/6152, https://github.com/Microsoft/vscode/issues/16824, https://github.com/Microsoft/vscode/issues/18684, https://github.com/Microsoft/vscode/issues/19014, https://github.com/Microsoft/vscode/issues/19398)