App.get_project_refs(): Use a set for deduplication

get_project_refs() deduplicates the walk results by scanning the result
list for each appended element, so the cost grows quadratically with the
number of projects, which is noticeable when walking a dev tree of 320
projects.

Track the already-seen projects in a set, and append an element only when
it is new. The resulting list is unchanged, and the membership test is O(1)
instead of O(n).

Signed-off-by: Jan Lindemann <jan@janware.com>
Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v0.86.1

src/python/jw/pkg/App.py

@@ -590,6 +590,7 @@ class App(Base):
         if isinstance(keys, str):
             keys = [keys]
         ret: list[str] = []
+        seen: set[str] = set()
         for key in keys:
             visited: set[str] = set()
             for name in projects:
@@ -597,9 +598,9 @@ class App(Base):
                 self.walk_project_deps(
                     rr, visited, name, sections, key, add_self, scope, names_only
                 )
-                # TODO: this looks like a performance hogger
                 for m in rr:
-                    if m not in ret:
+                    if m not in seen:
+                        seen.add(m)
                         ret.append(m)
         return ret