Don’t cause any file system file effects when trying to find an implicit workspace for a file

When looking for a workspace that can handle a file, we were creating full-fledged workspaces along the way, which we would then discard if they couldn’t handle the file being opened. This had multiple problems:
1. When background indexing is enabled, it caused semantic indexing of the workspace, which wrote files to a `.index-build` directory and was a waste of work
2. When background indexing is enabled, it caused package resolution, which also created a `.index-build` folder to be created
3. It caused a syntactic test index of the workspace, which was a waste of work.

To fix this, do multiple things:
1. When creating a workspace, add a check right after build system creation. This allows us to early exit if the build system can’t handle the file and prevents us from generating the `Workspace`, fixing (1) and (3)
2. Don’t call `reloadPackage` when creating a `SwiftPMWorkspace`. Instead, explicitly call `generateBuildGraph` once we committed to creating the workspace.

Sources/SKCore/BuildServerBuildSystem.swift

@@ -279,7 +279,7 @@ extension BuildServerBuildSystem: BuildSystem {
     return [ConfiguredTarget(targetID: "dummy", runDestinationID: "dummy")]
   }
 
-  public func generateBuildGraph() {}
+  public func generateBuildGraph(allowFileSystemWrites: Bool) {}
 
   public func topologicalSort(of targets: [ConfiguredTarget]) async -> [ConfiguredTarget]? {
     return nil