Drop what only a yielded library needed

After a library fetched by name yields to an explicit one, the components
no top-level library reaches any more are not converted either.

esphome/platformio/library.py

@@ -1510,6 +1510,7 @@ def convert_libraries(
     # same name, whichever round brought it in; the settle step applied that
     # rule to the names it could match up front
     explicit = {k: c for k, c in components.items() if not fetched_by_name(k)}
+    yielded = False
     for key in [k for k in components if fetched_by_name(k)]:
         node = nodes[key]
         match = _satisfying_key(node.pkgname, node.owner, key, nodes, explicit)
@@ -1517,10 +1518,22 @@ def convert_libraries(
             continue
         _LOGGER.debug("Library %s fetched by name yields to %s", key, match)
         del components[key]
+        yielded = True
         for other in nodes.values():
             if key in other.edges:
                 other.edges.remove(key)
                 other.edges.add(match)
+    if yielded:
+        # What only the yielded library needed is not built either
+        reachable: set[str] = set()
+        stack = [key for key in top_level if key in components]
+        while stack:
+            if (key := stack.pop()) in reachable:
+                continue
+            reachable.add(key)
+            stack.extend(edge for edge in nodes[key].edges if edge in components)
+        for key in [key for key in components if key not in reachable]:
+            del components[key]
 
     # Two graph nodes that resolve to the same component name (e.g. a package
     # referenced both bare and as ``owner/name``) are not deduplicated and can

tests/unit_tests/test_platformio_library.py

@@ -1471,7 +1471,9 @@ def test_by_name_fetch_yields_to_a_fork_pulled_in_later(tmp_path, monkeypatch) -
         {
             "esphome/A": {"name": "A", "dependencies": [{"name": "B"}]},
             "esphome/X": {"name": "X", "dependencies": [{"name": "C"}]},
-            "B": {"name": "B"},
+            # The upstream B pulls in D, which nothing else needs
+            "B": {"name": "B", "dependencies": {"esphome/D": "1.0.0"}},
+            "esphome/D": {"name": "D"},
             "C": {"name": "C", "dependencies": {"custom/B-fork": "1.0.0"}},
             "custom/B-fork": {"name": "B"},
         },
@@ -1482,7 +1484,7 @@ def test_by_name_fetch_yields_to_a_fork_pulled_in_later(tmp_path, monkeypatch) -
         _backend(emit=lambda c: emitted.append(c.name)),
     )
     assert [d.name for d in top[0].dependencies] == ["custom/B-fork"]
-    assert "B" not in emitted
+    assert sorted(emitted) == ["C", "custom/B-fork", "esphome/A", "esphome/X"]
 
 
 def test_registry_miss_resolves_again_as_a_git_source(tmp_path, monkeypatch) -> None: