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
We currently load the entire package before generating a `SwiftPMBuildSystem`. That means that the initialize request to SourceKit-LSP is blocked until the package has been loaded, preventing us from offering any sort of functionality, including syntactic functionality like formatting.
Decouple build system creation and build graph generation (aka. package loading for SwiftPM). We can operate with fallback build settings until the build graph has been loaded and reopen the document once the proper build settings are available.
rdar://126644596
