nix3-profile: make element names stable

Based off of commit 6268a45b650f563bae2360e0540920a2959bdd40

Upstream-PR: https://github.com/NixOS/nix/pull/9656
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Change-Id: I0fcf069a8537c61ad6fc4eee1f3c193a708ea1c4

src/libcmd/cmd-profiles.hh

@@ -35,7 +35,6 @@ constexpr int DEFAULT_PRIORITY = 5;
 struct ProfileElement
 {
     StorePathSet storePaths;
-    std::string name;
     std::optional<ProfileElementSource> source;
     bool active = true;
     int priority = DEFAULT_PRIORITY;
@@ -57,7 +56,7 @@ struct ProfileElement
 
 struct ProfileManifest
 {
-    std::vector<ProfileElement> elements;
+    std::map<std::string, ProfileElement> elements;
 
     ProfileManifest() { }
 
@@ -67,6 +66,9 @@ struct ProfileManifest
 
     StorePath build(ref<Store> store);
 
+    void addElement(std::string_view nameCandidate, ProfileElement element);
+    void addElement(ProfileElement element);
+
     static void printDiff(const ProfileManifest & prev, const ProfileManifest & cur, std::string_view indent);
 };