Allow project discovery for OTP (#141)

Summary:
Some clients, such as emacs eglot, do their own project discovery when a new file is opened, even from the target of a go to definition.

They then launch a fresh ELP server if they determine a different project root.

In the ELP server setup we preconfigure the OTP project root as known, but with a project manifest of None. This means go to def into an OTP file ends up with an uninitialized project in the new server.

This diff makes it so that if the project root is known, but has no project manifest, we do discovery on that root.

Pull Request resolved: https://github.com/whatsapp/erlang-language-platform/pull/141

Reviewed By: robertoaloi

Differential Revision: D89169465

Pulled By: alanz

fbshipit-source-id: db180d67df98fd1f1fd61a53a93f9adfa9e93ebd

crates/elp/src/project_loader.rs

@@ -100,8 +100,11 @@ impl ProjectLoader {
     ) -> Option<(ElpConfig, Result<ProjectManifest>, ProjectManifest)> {
         let mut path_it = path;
         loop {
-            if self.project_roots.contains_key(path_it) {
-                return None;
+            if let Some(value) = self.project_roots.get(path_it) {
+                return match value {
+                    None => Some(self.load_manifest(path_it)),
+                    Some(_) => None,
+                };
             }
             match path_it.parent() {
                 Some(parent) => path_it = parent,