Fix a race condition that could cause the build graph to not be generated when doing initial background indexing

We were making the initial `generateBuildGraph` call in `SourceKitLSPServer` from a `Task`. This means that `generateBuildGraph` could be executed after `waitForUpToDateBuildGraph` was called by `SemanticIndexManager`. Thus `waitForUpToDateBuildGraph` returned immediately and no files were background indexed.

Make `generateBuildGraph` immediately schedule a package reload for SwiftPM build systems instead of doing the hop through a `Task`, fixing the race condition.

rdar://135551812

Sources/BuildSystemIntegration/CompilationDatabaseBuildSystem.swift

@@ -133,7 +133,7 @@ extension CompilationDatabaseBuildSystem: BuildSystem {
     throw PrepareNotSupportedError()
   }
 
-  package func generateBuildGraph() {}
+  package func scheduleBuildGraphGeneration() {}
 
   package func waitForUpToDateBuildGraph() async {}