Language server lookup cache for file is corrupted when editing method

[muglug]

To reproduce:

1. Run ./psalm --clear-global-cache which clears the language server's cache
2. Open VS Code with the Psalm plugin
3. Open tests/AlgebraTest.php
4. Notice all hovers, go-to-definition etc. work as intended
5. Edit the body of the first method
6. Notice that hovers, go-to-definition etc. in other methods no longer work
7. Editing those other methods brings back hover and go-to-definition

cc @joehoyle as an FYI. This maybe didn't ever work? Not sure

[muglug]

ah ok this was broken in #5084

[muglug]

still a bit buggy

[muglug]

I think this is probably good to close again