"Show ppxed source" fails looking for .cmt file by the wrong name

Invoking the command in question pops this vscode error:

```
Unable to open 'common.ml': Cmt file does not exist
/home/chas/dev/project/_build/.lsp/Project__Common.cmt.
```

However, the .cmt file _is_ there, just with a different name, ` /home/chas/dev/project/_build/.lsp/project__Common.cmt` (note the case, "project__" vs. "Project__").

Renaming the cmt file in question to have the case being looked for by the plugin and re-running the "show ppxed source" command successfully pops open a ppx-expanded buffer.

Start by tracing the "Show ppxed source" command's cmt path and name resolution, using the reported uppercase and lowercase filenames as the reproduction. Confirm the fix by rerunning the command and verifying that it opens the ppx-expanded buffer when the existing lowercase cmt file is present.

Written by the indexing model from the issue text.