We previously waited for the initialization response from the build server during the creation of a `Workspace` so that we could create a `SemanticIndexManager` with the index store path etc. that was returned by the `build/initialize` response. This caused all functionality (including syntactic) of SourceKit-LSP to be blocked until the build server was initialized.
Change the computation of the `SemanticIndexManager` and related types to happen in the background so that we can provide functionality that doesn’t rely on the build server immediately.
Fixes#2304
