"Could not find the source file" error after deleting .qs file when no .csproj exists

[rmshaffer]

Describe the bug

In latest beta builds from master, the language server is creating a temporary project to compile and provide IntelliSense features for .qs files that do not have an associated .csproj file in the folder.

If one of these .qs files is deleted during a VS Code session, an error appears in the Problems tab, such as:
Could not find the source file "c:\QDK\RyanTest\TestPython\Test2.qs" to compile.

To Reproduce

Steps to reproduce the behavior:

1. Create a folder containing a couple of .qs files (but no .csproj).
2. Open that folder in VS Code.
3. Open the .qs files and wait for the Q# extension to finish loading (check the Output tab in VS Code).
4. Delete one of the .qs files.
5. Observe the error on the Problems tab. The error persists until VS Code is closed (but it does go away after restarting).

Expected behavior

No such error should occur when deleting a file.

System information

• Extension version 0.12.20090307-beta
• Windows 10, .NET Core 3.1