Add API for background preparation in SourceKit-LSP (#7540)

- Add an API to get the name of a target from SourceKit-LSP 
  - We will need this API to prepare a target for indexing.
- Add an API to get all the targets in a build description / modules
graph in a topological order
- We need this for background preparation for indexing so that we can
prepare and index lower-level targets first.

Author: ahoppen

Sources/PackageGraph/ModulesGraph.swift

@@ -101,6 +101,14 @@ public struct ModulesGraph {
     /// Returns all the targets in the graph, regardless if they are reachable from the root targets or not.
     public private(set) var allTargets: IdentifiableSet<ResolvedModule>
 
+    /// Returns all targets within the module graph in topological order, starting with low-level targets (that have no
+    /// dependencies).
+    package var allTargetsInTopologicalOrder: [ResolvedModule] {
+        get throws {
+            try topologicalSort(Array(allTargets)) { $0.dependencies.compactMap { $0.target } }.reversed()
+        }
+    }
+
     /// Returns all the products in the graph, regardless if they are reachable from the root targets or not.
     public private(set) var allProducts: IdentifiableSet<ResolvedProduct>
 

Sources/SourceKitLSPAPI/BuildDescription.swift

@@ -27,6 +27,9 @@ import struct PackageGraph.ModulesGraph
 public protocol BuildTarget {
     var sources: [URL] { get }
 
+    /// The name of the target. It should be possible to build a target by passing this name to `swift build --target`
+    var name: String { get }
+
     /// Whether the target is part of the root package that the user opened or if it's part of a package dependency.
     var isPartOfRootPackage: Bool { get }
 
@@ -46,6 +49,10 @@ private struct WrappedClangTargetBuildDescription: BuildTarget {
         return (try? description.compilePaths().map { URL(fileURLWithPath: $0.source.pathString) }) ?? []
     }
 
+    public var name: String {
+        return description.clangTarget.name
+    }
+
     public func compileArguments(for fileURL: URL) throws -> [String] {
         let filePath = try resolveSymlinks(try AbsolutePath(validating: fileURL.path))
         let commandLine = try description.emitCommandLine(for: filePath)
@@ -63,6 +70,10 @@ private struct WrappedSwiftTargetBuildDescription: BuildTarget {
         self.isPartOfRootPackage = isPartOfRootPackage
     }
 
+    public var name: String {
+        return description.target.name
+    }
+
     var sources: [URL] {
         return description.sources.map { URL(fileURLWithPath: $0.pathString) }
     }
@@ -110,4 +121,12 @@ public struct BuildDescription {
             return nil
         }
     }
+
+    /// Returns all targets within the module graph in topological order, starting with low-level targets (that have no
+    /// dependencies).
+    public func allTargetsInTopologicalOrder(in modulesGraph: ModulesGraph) throws -> [BuildTarget] {
+        try modulesGraph.allTargetsInTopologicalOrder.compactMap {
+            getBuildTarget(for: $0, in: modulesGraph)
+        }
+    }
 }

Sources/SourceKitLSPAPI/PluginTargetBuildDescription.swift

@@ -34,6 +34,10 @@ struct PluginTargetBuildDescription: BuildTarget {
         return target.sources.paths.map { URL(fileURLWithPath: $0.pathString) }
     }
 
+    var name: String {
+        return target.name
+    }
+
     func compileArguments(for fileURL: URL) throws -> [String] {
         // FIXME: This is very odd and we should clean this up by merging `ManifestLoader` and `DefaultPluginScriptRunner` again.
         let loader = ManifestLoader(toolchain: try UserToolchain(swiftSDK: .hostSwiftSDK()))