fix: fall back to lib/bs directory for deps without .sourcedirs.json

Prebuilt dependencies like @rescript/react have .cmt files in lib/bs/
but no .sourcedirs.json. The mtime-based detection introduced in
f178c45 returned None for these, causing the analysis to skip the
dependency entirely and breaking completions for ReactDOM, ReactEvent,
JsxEvent, etc. on macOS CI.

Author: nojaf

analysis/src/BuildSystem.ml

@@ -50,7 +50,7 @@ let getLibBs path =
     if mtime (sourcedirs lsp) > mtime (sourcedirs bs) then Some lsp else Some bs
   | true, false -> Some bs
   | false, true -> Some lsp
-  | false, false -> None
+  | false, false -> Files.ifExists bs
 
 let getStdlib base =
   match getRuntimeDir base with