Diagnostics do not work on symlinked files

Syntax highlighting does not work on symlinked files, but it does on original one.

Environment

• Operating System: Ubuntu 20.04
• JDK version: 13.0.7
• Visual Studio Code version: 1.63.2
• Java extension version: 1.3.0

Steps To Reproduce

1. Create a sample java project with a .java file and introduce an error to it (e.g. use HashMap<String, String> without importing HashMap).
2. You should see HashMap underlined with an error HashMap cannot be resolved to a type
3. Create a symlink of the file to somewhere outside the project (in ubuntu: ln -s <path to original file> <path somewhere outside src/main/java structure>)
4. The new file won't have HashMap underlined

Reproduce the issue with the listed Ubuntu, JDK, and VS Code setup, comparing diagnostics on the original and symlinked .java files. Trace how files outside the src/main/java structure are handled; done means the symlinked file shows the same diagnostic as the original.