LS: Deleted files are not disappearing from file/module tree

[mkaput]

Reproduction

Setup

1. scarb new repro
2. src/lib.cairo:

   mod fib;

3. src/fib.cairo:

   pub fn fib(mut n: u32) -> u32 {
       let mut a: u32 = 0;
       a;
   }

4. Ensure LS state is clean, everything is analyzed etc.
5. Make an error in src/fib.cairo, for example:

   pub fn fib(mut n: u32) -> u32 {
       let mut a: u32 = ; // syntax error
       a;
   }

Expected behaviour working with another scenario

1. Comment-out mod fib; in src/lib.cairo
2. Notice errors from fib.cairo disappear from Problems panel.

Buggy scenario

1. Keep mod fib; but delete the fib.cairo file entirely via VSCode.
2. Notice that errors from fib.cairo did not vanish!

Notes

• I have identified that this is independent of open_files tracking, fib.cairo is properly removed from it.
• But fib.cairo stays in file/module tree and thus lands in secondary_files set when computing diagnostics. It should disappear from this set upon deletion.