[SourceKitLSPAPI] Add a way to retrieve headers from a `BuildTarget

Author: xedin

Sources/SourceKitLSPAPI/BuildDescription.swift

@@ -29,8 +29,12 @@ internal import class PackageModel.UserToolchain
 public typealias BuildTriple = PackageGraph.BuildTriple
 
 public protocol BuildTarget {
+    /// Source files in the target
     var sources: [URL] { get }
 
+    /// Header files in the target
+    var headers: [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 }
 
@@ -52,7 +56,14 @@ private struct WrappedClangTargetBuildDescription: BuildTarget {
     }
 
     public var sources: [URL] {
-        return (try? description.compilePaths().map { URL(fileURLWithPath: $0.source.pathString) }) ?? []
+        guard let compilePaths = try? description.compilePaths() else {
+            return []
+        }
+        return compilePaths.map(\.source.asURL)
+    }
+
+    public var headers: [URL] {
+        return description.clangTarget.headers.map(\.asURL)
     }
 
     public var name: String {
@@ -92,6 +103,8 @@ private struct WrappedSwiftTargetBuildDescription: BuildTarget {
         return description.sources.map { URL(fileURLWithPath: $0.pathString) }
     }
 
+    var headers: [URL] { [] }
+
     func compileArguments(for fileURL: URL) throws -> [String] {
         // Note: we ignore the `fileURL` here as the expectation is that we get a command line for the entire target
         // in case of Swift.

Sources/SourceKitLSPAPI/PluginTargetBuildDescription.swift

@@ -39,6 +39,8 @@ struct PluginTargetBuildDescription: BuildTarget {
         return target.sources.paths.map { URL(fileURLWithPath: $0.pathString) }
     }
 
+    var headers: [URL] { [] }
+
     var name: String {
         return target.name
     }