Add unique element id for page links

WebsiteGenerator/Files/FileSystem.swift

@@ -57,6 +57,13 @@ final class FileSystem {
      */
     private var emptyPages: Set<String> = []
 
+    /**
+     All paths to page element folders, indexed by their unique id.
+
+     This relation is used to generate relative links to pages using the ``Element.id`
+     */
+    private var pagePaths: [String: String] = [:]
+
     /**
      The image creation tasks.
 
@@ -423,6 +430,13 @@ final class FileSystem {
         }
     }
 
+    func add(page: String, id: String) {
+        if let existing = pagePaths[id] {
+            log.add(error: "Conflicting id with \(existing)", source: page)
+        }
+        pagePaths[id] = page
+    }
+
 
     // MARK: Writing files